I. Introduction
Despite their non-binding character, the ripples of International Court of Justice (“ICJ”) advisory opinions are often felt strongly across the international and domestic legal orders. This will be evident to anyone who has followed R. (on the Application of Western Sahara Campaign UK) v The Commissioners for HMRC and the Secretary of State for the Environment, Food and Rural Affairs Footnote 1 and related litigation before EU courts over the years;Footnote 2 current effortsFootnote 3 to stop trade with and investment in Israel and its illegal settlements in Palestinian territory in line with the ICJ advisory opinion on the Legal Consequences Arising from the Policies and Practices of Israel in the Occupied Palestinian Territory, including East Jerusalem;Footnote 4 and the UK-Mauritius Agreement concerning the handover of the Chagos Archipelago to Mauritius (the “UK-Mauritius Agreement”).Footnote 5
Focusing on the UK-Mauritius Agreement, this article maps and highlights the normative effects of ICJ advisory opinions. At a time when the advisory function of international courts is resorted to more frequently than ever before, a better understanding of the potential of advisory opinions is needed.Footnote 6 Contrary to judgments in contentious proceedings which are final and binding on disputing parties,Footnote 7 ICJ advisory opinions are formally non-binding. This notwithstanding, as this article argues, their normative effects cannot be adequately captured by their non-binding character, or the common understanding that they merely constitute “advice” to the body requesting the opinion. Rather, their effects can be felt across the international legal order, within and outwith the UN system, and in different judicial and non-judicial fora.
To make this argument, the analysis is structured as follows. Section II provides a brief sketch of the background and context of the Chagos Archipelago handover, including the ICJ advisory opinion on the Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965,Footnote 8 the UK’s reaction to it and an overview of the UK-Mauritius Agreement. The aim is to highlight the shifting position of the UK from defiance of, to compliance with, its international obligations as determined by the court in the Chagos advisory opinion. While this policy shift can also be understood as an attempt to restore the UK’s standing in the international community,Footnote 9 the argument made here is that the normative effects of the Chagos advisory opinion, as expressed in a range of international fora, were instrumental in making the UK’s defiant position “increasingly untenable”.Footnote 10 Section III then sets the frame for understanding the normativity of ICJ advisory opinions, discussing their non-binding character, what follows and, more crucially, what does not follow from it. Section IV analyses two different terrains in which the normative effects of advisory opinions may be traced, returning to the Chagos advisory opinion as a case in point. The article’s overall argument is that focusing on their formally non-binding character belies the normativity and significance of ICJ advisory opinions across the international legal order. The final section concludes, reflecting on the capacity of the UK-Mauritius Chagos Agreement to finally right a long-standing international wrong.
Before proceeding with the analysis, it is important to clarify a preliminary point to more concretely delineate the question this article seeks to address. One way to understand the normative effects of ICJ advisory opinions is to view them through the lens of the Statute of the ICJ, art. 38(1)(d), namely as “judicial decisions” that constitute “subsidiary means for the determination of rules of law”. The International Law Commission (“ILC”) has indeed considered in its ongoing work on subsidiary means that advisory opinions of the ICJ are encompassed by Statute of the ICJ, art. 38(1)(d).Footnote 11 They may therefore be relevant for determining the “existence and content” of rules of international law,Footnote 12 such as when the ICJ determined that the right to self-determination crystallised as customary international law in the early 1960sFootnote 13 or when it found the status of this right in cases of foreign occupation to be ius cogens.Footnote 14 Connected to this, the ICJ is generally thought to have greatly contributed to the development of international law.Footnote 15 This includes the court’s advisory function, for example in relation to the legal framework governing reservations to treaties.Footnote 16 This article, however, is not concerned with this type of effect that an ICJ advisory opinion may produce, namely with the court’s identification of the existence and content of rules of international law generally, or its contribution to the development of new rules of law. Rather, the focus is on demonstrating and explaining the normative effects of the ICJ’s concrete answers to the questions posed to it in an advisory opinion request – the equivalent of the operative part of a judgment in contentious proceedings – particularly where, as in the Chagos advisory opinion, such answers concern the legal position (rights and obligations) of specific states.Footnote 17
II. The Chagos Archipelago Handover
In 2017, the UN General Assembly requested an advisory opinion from the ICJ under Charter of the UN, art. 96Footnote 18 and Statute of the ICJ, art. 65 on the detachment of the Chagos Archipelago from Mauritius. This came on the back of years of litigation before EnglishFootnote 19 and internationalFootnote 20 courts by both the Chagossians – the indigenous people who were forcibly displaced from Chagos by the UK and the US between 1965 and 1973 so that a UK–US military base could be established on Diego Garcia, the Archipelago’s largest islandFootnote 21 – and Mauritius, the former seeking reparations for their displacement and recognition of their right to return to their islands, and the latter seeking a declaration of illegality of the UK’s presence in the Archipelago and recognition of its own sovereignty over it. The UN General Assembly asked the court two questions: first, whether “the process of decolonization of Mauritius [was] lawfully completed” in 1968 given the detachment of Chagos from it and in light of relevant rules and principles of international law at the time; second, what were the “consequences under international law […] arising from the [UK’s] continued administration” of Chagos, including in relation to “[Mauritius’s] inability” to resettle the Chagossians.Footnote 22
The UK resisted the advisory opinion request, deeming it an improper path for bringing a bilateral sovereignty dispute before the ICJ without its consent.Footnote 23 It kept this stance throughout the proceedings, in which it argued that the ICJ should exercise its discretion to refuse to give an opinion for this reason.Footnote 24 Nevertheless, the court (and most states participating in the proceedings) understood the questions posed to it as pertaining to the General Assembly’s competence in matters of decolonisation and decided to grant the request.Footnote 25 In its opinion, it found that the process of decolonisation of Mauritius was not lawfully completed because the UK had excised the Chagos Archipelago from Mauritius three years before the latter achieved independence in 1968.Footnote 26 According to the court, at the time when Mauritius gained independence, customary international law, particularly the right to self-determination, prohibited the dismemberment of colonial territories.Footnote 27 In other words, colonised peoples had the right to self-determination with respect to the totality of a non-self-governing (i.e. colonised) territory.Footnote 28 Consequently, the ICJ found the UK to be engaged in an internationally wrongful act of a “continuing character” entailing its international responsibility and determined that it is obligated under international law “to bring to an end its administration of the Chagos Archipelago as rapidly as possible” so that Mauritius can “complete the decolonization of its territory” in accordance with the right of peoples to self-determination.Footnote 29 The court also found other states, particularly UN Members, to be “under an obligation to cooperate with the United Nations in order to complete the decolonization of Mauritius”.Footnote 30
In the aftermath of the Chagos advisory opinion, the UN General Assembly adopted Resolution 73/295, endorsing the court’s findings, affirming that Chagos “forms an integral part of the territory of Mauritius” and calling on the UK to “withdraw its colonial administration […] unconditionally within a period of no more than six months”.Footnote 31 The UK initially refused to accept the court’s determinations and opposed the resolution, maintaining its sovereignty claim over Chagos and highlighting both internationally and domestically the opinion’s non-binding character. A statement to the House of Commons by Sir Alan Duncan (then Minister of the Foreign, Commonwealth and Development Office) clearly captures the UK’s approach at the time: “An advisory opinion is advice provided to the United Nations General Assembly at its request; it is not a legally binding judgment. The Government have considered the content of the opinion carefully, however we do not share the Court’s approach […] we have no doubt about our sovereignty over the Chagos archipelago.”Footnote 32
According to the UK, even though they may “carry weight in international law”, ICJ advisory opinions are merely “advice” to the requesting body and, in this case, not very good advice at that.Footnote 33 The UK maintained the same attitude until late 2022, when it was announced that it had entered into negotiations with Mauritius on “the exercise of sovereignty” over the Chagos Archipelago.Footnote 34 In the Foreign Secretary’s statement announcing the commencement of negotiations there was no longer reference to the non-binding nature of the advisory opinion; instead, it was said that “[t]hrough negotiations, taking into account relevant legal proceedings, it is our intention to secure an agreement on the basis of international law”.Footnote 35
Two years later, on 3 October 2024, the two states announced that political agreement had been reached on concluding a treaty that would recognise Mauritius’s sovereignty over Chagos, with the UK continuing to administer Diego Garcia for the purpose of maintaining the UK–US military base located there.Footnote 36 The final treaty text which was signed and laid before the UK Parliament on 22 May 2025 provides that Mauritius “is sovereign over the Chagos Archipelago in its entirety” but that the UK is “authorise[d] […] to exercise the rights and authorities of Mauritius with respect to Diego Garcia in accordance with the terms of this Agreement” for 99 years (a period that can be further extended upon agreement).Footnote 37 While this is not a blank cheque and Mauritius will maintain a range of rights and jurisdictional competences in relation to the island, its maritime zones and natural resources amongst others,Footnote 38 the “rights and authorities” granted to the UK are all those that it “requires for the long-term, secure and effective operation of the Base”.Footnote 39 The Agreement contains extensive security arrangements for and around Diego Garcia,Footnote 40 while the UK is entitled to “authorise” the US to jointly run the military base.Footnote 41 In return, the UK has agreed to pay Mauritius a total of £3.4 billion in various sums over the life of the Agreement.Footnote 42 In relation to the Chagossians, who were forcibly displaced from their islands to establish the military base, the Agreement provides that, being sovereign, “Mauritius is free to implement a programme of resettlement on the islands…other than Diego Garcia”Footnote 43 and that the UK will “capitalise” to the amount of £40 million a trust fund to be established by Mauritius “for the benefit of Chagossians”.Footnote 44
Since its announcement, there has been significant opposition to the Agreement in the UK, including through the tabling of parliamentary motions to oppose the treaty’s ratification.Footnote 45 While much of domestic resistance comes from a place of misplaced imperial nostalgia,Footnote 46 a range of domestic and international actors have also criticised the Agreement for failing to respect Chagossian rights, including the right of return and the right to be consulted in the negotiation process. I return to this important issue in Section V. Considering the question of ratification, in the UK treaty-making is part of the Royal prerogative exercised by the Government,Footnote 47 with the Constitutional Reform and Governance Act 2010 (“CRAG”) setting out the process to be followed before a treaty can be ratified. Under CRAG, s. 20, the Government can proceed to ratification when a signed treaty has been laid before Parliament for 21 sitting days without either House objecting to it.Footnote 48 For the UK-Mauritius Agreement this period expired on 3 July 2025.Footnote 49 However, the Government have said that they would not proceed with ratification before passing the necessary implementing legislation, including because changes to UK law will have to be made in light of the treaty.Footnote 50 The Diego Garcia Military Base and British Indian Ocean Territory Bill has been brought forward for this purpose, but at the time of writing the bill’s fate seems unclear.Footnote 51
Nevertheless, the next two sections analyse how and why the UK went from complete defiance of the advisory opinion to negotiating and concluding a treaty with Mauritius to hand over one of its last colonies.
III. Setting the Frame: The Non-Binding Character of Advisory Opinions and Its (Non-)Implications
There is no doubt that ICJ advisory opinions have formally no binding force. The very concept of an “advisory opinion” implies that “they constitute advice, namely they do not legally bind either the requesting entity or any other body or State to take any specific action pursuant to the opinion”.Footnote 52 As the court itself has affirmed, its “reply” to an advisory opinion request “is only of an advisory character: as such it has no binding force”.Footnote 53 Additionally, as stated in the advisory opinion on the Legality of the Threat or Use of Nuclear Weapons, an advisory opinion is not meant to “settle – at least directly – disputes between States, but to offer legal advice to the organs and institutions requesting the opinion”.Footnote 54 As such, an opinion is not given to states, but to the requesting body and granting a request “represents [the ICJ’s] participation in the activities of the” UN as its principal judicial organ.Footnote 55 This way, the court “lends its assistance to the General Assembly”, or any other body authorised to request an advisory opinion, “in the solution of a problem confronting it”.Footnote 56 While this does not imply that the requesting body is bound by the court’s opinion, it is understood that “as a matter of institutional comity, […] [it] has a duty to take the Court’s opinion duly into account”.Footnote 57 The UN General Assembly, as the most common requesting body,Footnote 58 has always endorsed the opinions the ICJ has delivered upon its requestsFootnote 59 and has undertaken action for their operationalisation, including by establishing subsidiary organs and follow-up mechanisms.Footnote 60
Still, it may be asked, looking beyond their formally non-binding character, what are other normative effects of advisory opinions, and of UN General Assembly resolutions endorsing them, that may persuade a defiant state to act in conformity with the court’s pronouncements. Indeed, in the House of Commons, the point has often been made that by concluding the UK-Mauritius Agreement the Government is “surrendering British sovereign territory […] [not] because of a binding legal verdict”Footnote 61 and that it “[is] in favour of a Court judgment that was advisory”.Footnote 62 This can be answered on two different levels. The first is addressed immediately below, while the second is analysed in Section IV.
First, in the decentralised international order the absence of a court with competence to issue binding decisions on a specific legal question or a dispute between two states (or other actors) does not detract from the existence of relevant obligations under international law or their potential breach by a state. It is in fact common for many international law violations to be identified by other states in a decentralised manner, unilaterally or through multilateral fora, such as international organisations. As the ILC Articles on Responsibility of States for Internationally Wrongful Acts confirm, international responsibility arises automatically from the commission of an internationally wrongful act, namely from conduct attributable to the state that is in breach of that state’s international obligations.Footnote 63 Such responsibility may then be invoked by the injured state, as well as other states in certain circumstances,Footnote 64 in a variety of ways, such as by presenting a claim to the responsible state through diplomatic channels.Footnote 65 As the ICJ had occasion to confirm in The Gambia v Myanmar, instituting legal proceedings before an international court is one way to invoke a wrongdoing state’s responsibility,Footnote 66 but certainly not the only way. More crucially still, the secondary obligations of cessation and non-repetition, and the obligation to make reparation, arise immediately from the wrongful act; they are a corollary of the state’s international responsibility.Footnote 67
Against this background, there may often be situations where there is (more or less reasonable) disagreement as to the specific contents of a rule, the legal consequences arising from certain conduct (in other words, whether certain conduct violates a rule of international law), or as to how a rule applies in specific circumstances. In such cases, a pronouncement by an international court (binding or not) can confirm, clarify or strengthen one state’s position regarding another’s violation of its obligations, adding in the case of a binding judgment a further obligation that the responsible state will be violating if it remains non-compliant.Footnote 68 However, judicial pronouncements are not, as such, necessary to either identify the violation and invoke the wrongdoing state’s responsibility, or for triggering the latter’s obligations to cease the unlawful act and make reparation. In that sense, international law is fundamentally of a “horizontal interpretive nature”.Footnote 69 The current UK Attorney General has recognised this in his instruction to government lawyers that, when advising Ministers on questions of international law: “Policies or actions which have little or no chance of being tested before a court and which are assessed as carrying a high risk under international law should be scrutinised very carefully.”Footnote 70 The claim then that the UK is not obligated to return the Chagos Archipelago to Mauritius because the Chagos advisory opinion and/or UN General Assembly Resolution 73/295 endorsing it are non-binding is wrong because the UK’s obligation to withdraw from the Archipelago stems from the unlawfulness of its conduct under international law, and particularly its violation of the right to self-determination.
At the same time, and in this decentralised setting, the normative authority of advisory opinions, notwithstanding their lack of binding force, can be explained as follows.Footnote 71 While they constitute advice to the requesting body, as Pratap observed, advisory opinions “are not legal advice in the ordinary sense” – that is, in the way that counsel would advise a client.Footnote 72 They are rather pronouncements of a court in the exercise of its judicial function; they are “jurisdictional” acts.Footnote 73 In performing its advisory function, as in contentious proceedings, the ICJ “identif[ies] the abstract field in which the rules apply, their application to concrete situations, and/or the legal consequences flowing from their application”.Footnote 74 In other words, the court’s advisory opinions state the law, in the same way that its pronouncements in contentious cases do.Footnote 75 Their legal findings are considered “definitive”Footnote 76 and, for this reason, as others have remarked, they “may in effect be dispositive of the issue submitted”.Footnote 77
Further support for the argument advanced here can be found in the work of the Informal Inter-Allied Committee of Experts, which was convened in 1943 to make recommendations on the future of the Permanent Court of International Justice (“PCIJ”). When considering the PCIJ’s advisory function and whether it should be retained in the future, the experts’ understanding was that despite their formally non-binding character, advisory opinions offered “a procedure whereby the legal rights and position of the parties can be determined or at any rate assessed” – an opportunity for states “to ascertain their legal position” and for “the legal rights of the parties […] [to be] definitely ascertained” and a “form of disposal [of existing disputes] by legal means”.Footnote 78 In essence, the Committee understood the task of the court when granting advisory opinions, as “giving advice as to what the law is in relation to a defined issue or set of facts”.Footnote 79 This capacity of advisory opinions to dispose of specific legal questions is also reflected in the court’s recent consideration in its advisory opinion in Obligations of Israel in relation to the Presence and Activities of UNRWA and Third States in and in relation to the Occupied Palestinian Territory of whether it should have declined to grant the request so as not to “prejudg[e] elements of a pending contentious case”.Footnote 80 In rejecting this argument, the court did not refer to the non-binding character of its opinion, or to the latter’s lack of otherwise formal status, but rather distinguished the advisory proceedings from the pending contentious proceedings on the basis of their “subject-matters”.Footnote 81
Additionally, as others have argued, even though not formally binding, ICJ advisory opinions are binding on other UN organs in the sense that, if the latter “are obliged, or deliberately choose, to adopt a legal solution” to the issue submitted to the court, they will have to abide by the court’s pronouncements on the law.Footnote 82 While today this view may be thought to stretch the normativity of advisory opinions beyond orthodox understandings, this position was actually supported by various states in the UN General Assembly in relation to certain early advisory opinions of the court.Footnote 83 Pratap mentions the examples of Suzanne Bastid and Sir Gerald Fitzmaurice, international law experts and delegates of France and the UK in the General Assembly respectively, putting forward the view that the General Assembly was not entitled to second-guess or review the court’s conclusions on the points of law decided, nor could it “be argued that the opinion of the Court was wrong from a legal standpoint”.Footnote 84 Connected to this point, it has also been considered that ICJ advisory opinions “embody the official legal position of the [UN] on the legal question answered by the Court”.Footnote 85
Being judicial pronouncements, and those of the principal judicial organ of the UN,Footnote 86 ICJ advisory opinions then carry normative authority. Some scholars have conceptualised this authority as stemming from “persuasiveness”, which is linked to the court’s substantive authority, its expertise and the quality of its reasoning.Footnote 87 The argument made here is that persuasiveness, while valuable, does not exhaust how we should understand the normative authority of advisory opinions. This is to say that even though advisory opinions can be (and often are) highly persuasive, they are also definitive statements of what the law is. In other words, while they do not create new obligations for states (on account of the fact that they are non-binding), they definitively state what the existing obligations are, including by identifying how the law applies in concrete settings and what legal consequences follow from specific courses of conduct.Footnote 88 The ICJ, requesting bodies and states have treated advisory opinions so.Footnote 89 The ICJ itself has considered its past opinions as having precedential valueFootnote 90 in relation to their legal pronouncements, as well as their specific findings and determinations, and has relied on them in subsequent cases.Footnote 91 Domestic and supranational courts have also relied on them as authorities, not only when identifying and interpreting rules of international law, but also in relation to how such rules have been determined by the ICJ to apply in specific cases.Footnote 92
States have also recognised the normativity of the court’s opinions and have relied on the court’s pronouncements and findings in subsequent cases. A good example is afforded by the UNRWA advisory proceedings. The resolution adopted by the UN General Assembly requesting the court for an advisory opinion in this case, specifically “reaffirm[ed]” and “recall[ed]” findings and determinations that the court made in its advisory opinion on Israel’s Policies and Practices.Footnote 93 It also “[d]emand[ed] that Israel comply without delay with all of its legal obligations under international law, including as set out by the International Court of Justice”.Footnote 94 Additionally, from the 45 entities participating in the UNRWA advisory proceedings (41 states, the UN Secretary General and three intergovernmental organisations), all but four relied on the advisory opinion on Israel’s Policies and Practices in their written statements,Footnote 95 to support their arguments on a variety of legal points, including the territorial scope of the advisory opinion request,Footnote 96 the status of Israel as occupying power in the Occupied Palestinian Territory (“OPT”),Footnote 97 the illegality of Israel’s occupation and presence in the OPT,Footnote 98 Israel’s obligations as an occupier under international humanitarian lawFootnote 99 and the status, scope and contents of the Palestinian people’s right to self-determination.Footnote 100 What is more, as evidenced from the language used when relying upon the court’s prior determinations, many of the participants understood the relevant points of law as having been definitively decided upon by the court, namely as having been “effectively disposed” of.Footnote 101 In earlier times, too, even states that had opposed requesting the ICJ for an advisory opinion and/or disagreed with the court’s findings, and which were also sometimes specifically affected by the opinion given, nevertheless accepted that the court’s determinations of the law were authoritative and had to be followed.Footnote 102
To reiterate, the argument made here is that while advisory opinions do not themselves create new obligations for states or other actors, they constitute definitive pronouncements of the scope and content of existing rights and obligations. The point then is not whether advisory opinions are binding or not; they are not. The point is rather that binding force and normative force are not co-extensive and that the normative force of advisory opinions stems from their being judicial pronouncements that state the law. They make legal determinations, identifying the applicable legal framework and the application of rules in concrete circumstances, often also drawing legal consequences from such application for states and other actors. The effects of this normativity are then felt across other international, judicial and nonjudicial, fora, as discussed in the next section.
Before proceeding to this, however, one last point should be made going back to the decentralised nature of the international legal order. As discussed earlier in this section, in international law the identification of violations and the invocation of responsibility happen in a horizontal and decentralised manner. The same is true for enforcement. An injured state seeking to induce compliance by the responsible state with its obligations to cease an unlawful act and make reparation may resort to countermeasures, namely acts that are themselves unlawful (i.e. they breach rights of the responsible state or obligations owed to it under international law) but permitted as limited, temporary, reversible and proportionate measures to enforce the law.Footnote 103 It is possible that when taking countermeasures a state may have erred, for one reason or another, in its assessment that another state has breached international law and therefore bears international responsibility. This risk however disappears, where the ICJ has made relevant legal determinations in an advisory opinion, outlining the law as it applies in a specific context, declaring a certain situation lawful or unlawful and making factual findings.Footnote 104 An advisory opinion thus provides a justificatory basis on which injured and other statesFootnote 105 may rely in taking countermeasures, buttressing their claims with the normative force of its legal findings.Footnote 106
IV. Radiating Effects, or How Not to Lose Control over the Electromagnetic Spectrum
All this brings the analysis to the second level on which arguments that the UK is not obligated to return the Chagos Archipelago to Mauritius because the Chagos advisory opinion is not binding can be answered. And this is that the normative effects of an advisory opinion, particularly its concrete findings and determinations, will influence a variety of other international legal processes and mobilise developments across different institutional settings. To demonstrate this, this section traces the normative effects of ICJ advisory opinions in two different terrains: first, within the broader UN system; second, in subsequent contentious proceedings before international courts and tribunals.
The Chagos advisory opinion and its aftermath, including the UK-Mauritius Agreement, provide an excellent illustration of this because the UK, as discussed in Section II, opposed the advisory opinion from the start and maintained its disagreement with the court’s findings, and its sovereignty claim over Chagos, for some time. Additionally, the Explanatory Memorandum that accompanies the UK-Mauritius AgreementFootnote 107 provides good insight into (at least some of) the reasons that led the UK to reach an agreement with Mauritius to resolve this issue, further illuminating the analysis that follows.
A. Within the United Nations System
First, the normative effects of advisory opinions radiate across the broader UN “family”, consisting not only of the organisation’s primary and subsidiary organs, but also of 17 specialised agencies and other international organisations and bodies that are connected to the UN with special agreements. Here the focus is on the effects that an advisory opinion may have on the work of other UN organs, as well as specialised agencies. As background, the UN comprises six principal organs and numerous subsidiary organs pursuant to Charter of the UN, art. 7. UN specialised agencies, on the other hand, are international, intergovernmental organisations in their own right but are also part of the UN system in accordance with the terms of Charter of the UN, arts. 57 and 63. Their special relationship to the UN is created by means of an agreement (the “relationship agreement”) concluded between each of these international organisations and the Economic and Social Council (one of the UN’s principal organs) “subject to approval by the General Assembly”.Footnote 108 Their activities span the fields of economic, social, cultural, educational, technical and health matters.Footnote 109
As discussed in Section II, the General Assembly in response to the Chagos advisory opinion adopted Resolution 73/295, which affirmed the ICJ’s findings that the Chagos Archipelago had been illegally excised from Mauritius, that consequently the decolonisation of the latter had not been lawfully completed and that the UK was under an obligation to withdraw from the Archipelago as soon as possible.Footnote 110 In addition to this, Resolution 73/295 “call[ed] upon” both the UN and its specialised agencies to recognise and treat Chagos as “an integral part of Mauritius” and to not “giv[e] effect to any measure taken by or on behalf of, the ‘British Indian Ocean Territory’”.Footnote 111 A year later, the UN Secretary General reported to the General Assembly that in implementing Resolution 73/295, the Secretariat had changed how it designates Chagos on maps it produces.Footnote 112 According to the same report, other UN bodies and specialised agencies had started considering what the Chagos advisory opinion and Resolution 73/295 meant for their own activities.Footnote 113 As a result of such processes, the Office of the High Commissioner of Human Rights (“OHCHR”) informed Mauritius that it should proceed to include information relating to the Chagos Archipelago in its state party reports to human rights treaty bodies and to the Human Rights Council for the Universal Periodic Review.Footnote 114 The implication of this is that the OHCHR, constituting the office with “principal responsibility” for UN human rights activitiesFootnote 115 and acting as the secretariat for the Human Rights Council and UN human rights treaty bodies, will now consider the Chagos Archipelago as falling under the jurisdiction of Mauritius for purposes of human rights protection. Additionally, the Universal Postal Union (“UPU”), a UN specialised agency with competence relating to postal services, decided in 2021 to “no longer register, distribute or forward postage stamps issued by the ‘British Indian Ocean Territory’”.Footnote 116 While developments such as the latter may at first sight seem of limited significance, things become more serious when one considers more closely the significant technical activities undertaken by a range of UN specialised agencies.
This seems to have been of some concern to the UK, motivating its efforts to reach a negotiated settlement with Mauritius over Chagos. Both in ParliamentFootnote 117 and in the Prime Minister’s speech announcing the conclusion of the UK-Mauritius Agreement,Footnote 118 particular reference has been made to the UK’s ability to control the electromagnetic spectrum over Diego Garcia where the UK–US military base is located. To give more context, the electromagnetic spectrum consists of the entire range of electromagnetic radiation or waves.Footnote 119 Part of it – the radio spectrum that is used to transmit information and communications – is administered by the International Telecommunication Union (“ITU”), another UN specialised agency.Footnote 120 While each state has the sovereign right to manage the spectrum within its territory,Footnote 121 the ITU through its Radiocommunication Sector administers, coordinates and allocates use of radio frequencies internationally, including for use by satellite services, in accordance with its Radio Regulations.Footnote 122 Under this international agreement, binding on both the UK and Mauritius, no radio station can operate within the territory of a state without authorisation which is granted through a licensing system by the relevant authorities.Footnote 123 Additionally, under both the Radio Regulations and ITU Constitution, art. 45, states undertake “not to cause harmful interference” to radio services or communications of other states operating in accordance with the Radio Regulations.Footnote 124 The ITU Radiocommunication Bureau via its Director may “assist in the resolution of cases of harmful interference, at the request of one or more interested administrations”, conduct investigations and present draft recommendations to the ITU Radio Regulations Board.Footnote 125 The latter has been vested with competence to consider such reports concerning unauthorised radio transmissions and incidents of harmful interference, and to make recommendations to the authorities concerned.Footnote 126
This mechanism has been used before. In 2023, Iran complained that Starlink Satellite Services was engaged in unauthorised and therefore illegal transmissions within its territory contrary to ITU Radio Regulations, art. 18.1. Following up on the complaint, the ITU Radio Regulations Board invited both Iran and the administrations connected to Starlink, namely Norway as the primary authority and the US as an associated administration (the state where Starlink’s parent company, SpaceX, is located), to provide more information. Based on the information received, the Radio Regulations Board concluded that such unauthorised transmissions were indeed taking place and “requested [Norway and the US] to comply with” ITU Radio Regulations, art. 18.1 and other relevant ITU rules “by taking immediate action to disable Starlink terminals operating within the territory of” Iran.Footnote 127 Absent action to fully comply with the Board’s request by the two administrations, the Board continued to call upon Norway and the US to do so in firmer terms,Footnote 128 most recently acceding to Iran’s request to activate the mechanism under ITU Resolution 119 to publicise information on the case, and on Norway’s and the US’s non-compliance with their ITU obligations, on the ITU website.Footnote 129
Against this background, were Mauritius to complain that unauthorised radio transmissions are taking place in the Chagos Archipelago, it is possible that, in deciding which state is the relevant administration for granting authorisations, the UK (and other states to which radio or satellite operators are registered) may be faced with an adverse decision of the Radio Regulations Board. As the Chagos Archipelago constitutes “integral part” of the territory of Mauritius in accordance with General Assembly Resolution 73/295 and the UK’s administration of the Archipelago has been determined by the ICJ to constitute a continuing illegal act, Mauritius would be the only competent authority to authorise radio and satellite transmissions with the territory under ITU Radio Regulations, art. 18.1. In a different scenario, were Mauritius to interfere with or seek to block such unauthorised transmissions, the UK’s argument that this constitutes harmful interference under ITU rules would not stand, if a decision was made that such transmissions were not authorised by the proper administration and therefore were not operating “in accordance with […] the Radio Regulations”.Footnote 130 While the constitution of the ITU includes a carve-out for military radio installations,Footnote 131 this exemption is narrow, does not include governmental radio installations generally and may not cover military communications that, for instance, take place through leased commercial satellites.Footnote 132
Additionally, even though the ITU Radio Regulations Board may only issue recommendations to concerned Member States, the Plenipotentiary Conference – the ITU’s highest organFootnote 133 – can take decisions on any “telecommunication questions” as necessary and “take measures in connection with” agreements the ITU has concluded with other international organisations.Footnote 134 In light of this, the Conference may decide, following on the steps of the UPU and the OHCHR, to treat Chagos for telecommunications purposes as part of Mauritius’s territory in line with Resolution 73/295. What is more, both the UK and Mauritius are parties to the ITU Optional Protocol on the Compulsory Settlement of Disputes which provides for ad hoc compulsory arbitration in case of telecommunication disputes arising under ITU rules.Footnote 135 An arbitral tribunal constituted under this Protocol to decide a dispute between Mauritius and the UK concerning radio transmissions in the Chagos Archipelago may take as a “given”, as discussed further in Section IV(B) below, that the UK is illegally administering the Archipelago and that it constitutes part of Mauritius’s territory.
This analysis leads to the crucial question of the status of UN General Assembly Resolution 73/295, which endorsed the Chagos advisory opinion and instructed UN organs, specialised agencies and other international organisations to treat the Archipelago as part of Mauritius, vis-à-vis both UN organs and bodies and specialised agencies. For UN Member States, UN General Assembly resolutions are generally non-binding, except when the Assembly acts in certain budgetary or other institutional matters.Footnote 136 When it comes to their status vis-à-vis other UN organs, however, the answer to this question is not as simple. While the legal nature of acts of the General Assembly addressed to the Secretariat (another principal organ of the organisation) is difficult to qualify,Footnote 137 especially in the absence of any formal or informal hierarchy between the two organs,Footnote 138 there is established practice of the Secretariat following instructions and directives of the General Assembly.Footnote 139 Additionally, according to a 1973 legal opinion of the Secretariat, “the policies and restraints contained in the resolutions […] constitute directives with which those who act under the authority the General Assembly, or of other principal organs of the United Nations, are bound to comply”.Footnote 140 This is irrespective of whether “such resolutions are considered legally binding by States”, with “United Nations organs […] bound to apply [them] to their own actions”.Footnote 141 This understanding is grounded in the functional division of powers between the different UN organs, with the UN Secretariat and the Secretary-General as chief administrative officer of the organisation,Footnote 142 having a supportive role in relation to the work of other principal organs and especially in the status of the General Assembly as the “principal deliberative organ” of the organisation.Footnote 143 In that sense, the latter expresses the official view of the organisation with which other organs are bound to comply.Footnote 144 Indeed, in modifying the designation of the Chagos Archipelago on its maps, the Secretary-General framed the Secretariat’s actions as being “in line with Resolution 73/295”.Footnote 145
Specialised agencies such as the ITU, on the other hand, have undertaken in their relationship agreements with the UN the obligation to submit to their competent organs for consideration recommendations made to them by the UN, to consult with the UN on such recommendations and report on actions taken in relation to them.Footnote 146 These obligations are underlined by the more general obligation of cooperation that structures the relationship between the UN and the specialised agencies. While in the past, some specialised agencies refused to follow UN General Assembly resolutions as non-binding on them,Footnote 147 it is not unlikely that based on the outlined legal framework, the ITU, like the UPU, may consider authoritative the ICJ’s determination that the UK’s presence in Chagos is illegal and decide that as determined by the UN General Assembly it should not recognise the UK’s authority to act on behalf of the territory. While the ITU is only one out of 17 specialised agencies, this discussion demonstrates how the close connections between the ICJ as the principal judicial organ of the UN, the political organs of the organisation espousing its opinions and the network of other UN organs and specialised agencies can augment and amplify the normative effects of advisory opinions.
B. In Subsequent Contentious Proceedings
The second terrain on which the normative effects of advisory opinions are expressed is subsequent contentious proceedings before other international courts and tribunals, or even the ICJ itself in another case. Before analysing their importance from this perspective, it is necessary to restate a preliminary, but fundamental, point regarding international adjudication, namely that it is based on consent. It is for this reason, namely that a state is only subject to the jurisdiction of an international court if it has given its consent,Footnote 148 that in the international legal order there may not always be a court with competence to hear specific disputes or claims, as alluded to in Section III. The implications of the consent principle are (at least) twofold.
First, when consent is given, this may be done in a limited manner, only for some types of disputes or complaints arising under a specific treaty. There are indeed many international agreements that contain such clauses granting consent to third-party dispute settlement for disputes arising under the particular treaty and concerning its “interpretation or application”.Footnote 149 In such cases, international tribunals have held consistently that their subject-matter jurisdiction is limited to deciding claims concerning that treaty, refusing to entertain claims falling outside of its scope.Footnote 150 While in theory this may sound straightforward, things are more complex in practice as states seek to utilise such clauses to litigate disputes that do not fit neatly into them.Footnote 151 One especially tricky question is whether, in deciding a claim over which it has subject-matter jurisdiction, an international court may make determinations relating to an issue which would not normally fall within its jurisdiction but is incidental to the main claim. When grappling with this problem, international tribunals have sometimes refused to exercise jurisdiction where they would have to decide an incidental issue which is not ancillary to the claim over which they have jurisdiction, but rather constitutes the main point of contention between the parties.Footnote 152 To illustrate this point, an arbitral tribunal constituted under the UN Convention on the Law of the Sea (“UNCLOS”) decided in the Chagos Marine Protected Area arbitration (preceding the Chagos advisory opinion) that it had no jurisdiction over two out of the four claims submitted by Mauritius against the UK, because they centred upon the sovereignty dispute over which it did not have subject-matter jurisdiction.Footnote 153
Second, according to the indispensable third-party principle an international court cannot exercise jurisdiction which it may possess over a dispute when a third state’s “legal interests” or its international responsibility “would form the very subject-matter of the decision”Footnote 154 and that third party has not granted its consent. This does not mean that adjudication is precluded in all cases where the interests of another state may be affected. As it has been observed, while the indispensable third-party principle is well-established in international law, practice shows that the threshold for applying it is relatively high and actual instances of its application are rare.Footnote 155 There are also certain situations where it is accepted that the principle may not be engaged, namely where the third state has essentially abandoned its interestsFootnote 156 or when other competent organs have already determined the question relating to the third state in an authoritative manner.Footnote 157 In the latter case, the tribunal concerned could take their decisions as “givens”.Footnote 158 Both these implications of the consent principle, that is (1) limited subject-matter jurisdiction and (2) the indispensable third-party principle, may be affected by the existence of a prior advisory opinion. Chagos is again a very good example here.
A little more than a year after the advisory opinion was delivered and Resolution 73/295 was adopted, Mauritius resorted to the UNCLOS dispute settlement provisions for a second time, instituting proceedings against the Maldives for the delimitation of their maritime boundary in the Indian Ocean.Footnote 159 Mauritius claimed that as it possesses sovereignty over the Chagos Archipelago – the coastal territory opposite the Maldives in the Indian Ocean – the Maldives had to negotiate with them the boundary for separating their overlapping maritime entitlements. The Maldives disagreed on the basis that a sovereignty dispute existed between the UK and Mauritius regarding Chagos, raising a series of jurisdictional objections on this basis.Footnote 160 More specifically, it argued that the UK was an indispensable third party to the proceedings and hence the tribunal could not exercise its jurisdiction without its consent; it also claimed that under the UNCLOS the tribunal did not have subject-matter jurisdiction to decide the sovereignty dispute itself. According to the ITLOS Special Chamber that heard the case both these objections turned on the legal status of the Chagos Archipelago and on whether a sovereignty dispute really did exist between the UK and Mauritius.Footnote 161
Reasoning on this issue, the Special Chamber considered the findings of the ICJ in the Chagos advisory opinion to “have unmistakable implications for the [UK’s] claim to sovereignty”Footnote 162 and that the ICJ’s “determinations do have legal effect”.Footnote 163 The Special Chamber espoused thus the distinction between an advisory opinion’s lack of binding force and its normative force,Footnote 164 as analysed in Section III. In light of this distinction, the Special Chamber found that “the legal status of the Chagos Archipelago has been clarified by the advisory opinion”,Footnote 165 for the ICJ had determined that the UK’s presence in the Chagos Archipelago was illegal and therefore only Mauritius was entitled to claim sovereignty over Chagos.Footnote 166 This was further “strengthen[ed]” by Resolution 73/295 endorsing the opinion, given the General Assembly’s competence on matters of decolonisation.Footnote 167 What is obvious from the Special Chamber’s reasoning is that the existence of the advisory opinion was crucial in deciding the matter in this way. It was this, namely that it “ha[d] the benefit of prior authoritative determination […] by [a] judicial body”, that distinguished for the Chamber this case from another, where one of the disputing parties had made a similar but unsuccessful argument as to the absence of a sovereignty dispute based only on General Assembly resolutions.Footnote 168
The Chagos advisory opinion allowed the Special Chamber to bypass both the indispensable third party issue and the objection concerning its limited subject-matter jurisdiction. In this light, it seems that the normativity of the court’s statements, as judicial pronouncements that state the law, had the effect of disposing of the disputed question of sovereignty, which plausibly existed before (as in Chagos Marine Protected Area) but not after the advisory opinion had been delivered. The opinion, in the words of the Special Chamber, had rendered the UK’s claim of sovereignty “a mere assertion”,Footnote 169 which did not have jurisdictional implications. Some scholars have criticised the Chamber’s reasoning on this point and its reliance on the “legal effects” of the advisory opinion, for essentially treating the court’s pronouncements in the Chagos advisory opinion as res judicata.Footnote 170 This criticism is based on the understanding that to grant “legal effect” to the findings of the Chagos advisory opinion means to “alter the rights or obligations of international legal subjects”.Footnote 171 But viewed through the lens of Section III of this article, the court’s findings in the Chagos advisory opinion did not create any new obligations for the states concerned, determining rather the content and scope of their existing rights and obligations under international law.
In that the Chamber’s treatment of the opinion is not unprecedented. The UK High Court, in a case concerning the lawfulness of the UK and the EU dealing with Morocco on behalf of the territory of Western Sahara, relied on the 1975 Western Sahara advisory opinion as dispositive of the question of whether Morocco had sovereignty links with that territory. According to the High Court, although “Morocco may claim” sovereignty over Western Sahara, its claim was not “recognise[d]” generally and “has been examined by the [ICJ]”.Footnote 172 Because the Western Sahara advisory opinion had “considered and rejected” Morocco’s argument that it enjoyed sovereignty over Western Sahara on the basis of such links pre-existing Spanish colonisation, the High Court considered that Morocco could not rely on this basis as grounding its sovereignty claim.Footnote 173
Therefore, the existence of an ICJ advisory opinion definitively determining certain points of law can be one way to “avoi[d]”Footnote 174 incidental issues or the indispensable third party question, clearing the path for an international court to exercise its jurisdiction in contentious proceedings. In light of this, it is not unlikely, as the Explanatory Memorandum accompanying the UK-Mauritius Agreement recognises, that faced with another contentious case brought by Mauritius under UNCLOS, or before the ICJ on the basis of another treaty, the UK may find itself faced with a binding judgment that takes the fact that there is no longer a sovereignty dispute in relation to the Chagos Archipelago as a given. At which point, even the argument that no international court has ever found in a binding judgment against the UK that the UK lacks sovereignty over Chagos would have to be abandoned.
An avenue to bring a contentious case implicating the question of sovereignty to the ICJ indeed exists. Both the UK and Mauritius are parties to the Indian Ocean Tuna Commission (“IOTC”) Agreement which contains in Article XXIII consent to ICJ jurisdiction.Footnote 175 While IOTC Agreement, Article XXIII devises a complex procedure that must be followed before a state party refers a dispute relating to the Agreement to the ICJ, Mauritius has challenged since 2021 the UK’s entitlement under IOTC Agreement, Article IV(a)(i) to participate in the IOTC as a coastal state, given its lack of sovereignty over Chagos.Footnote 176 If the dispute persisted, Mauritius could utilise Article XXIII to bring a contentious case to the ICJ. In this scenario, the ICJ would be faced with the question of whether its (limited) subject-matter jurisdiction under the IOTC Agreement encompasses the question of sovereignty. But it is a reasonable assumption that it may also utilise the Chagos advisory opinion as discussed above to avoid the issue. An advisory opinion can thus acquire “binding” teeth when its normativity affects in such a way the reasoning and, eventually, the findings of international tribunals in contentious proceedings.
V. Concluding Thoughts: International Compliance At Last; But Is It Really?
This article has provided a thicker account of the normativity of ICJ advisory opinions, arguing that despite their formally non-binding character their normative effects radiate across the international legal order. It has also shown how their determinations may be put to practical effect through a variety of international legal processes, which eventually may leave the state concerned with no option but to act in accordance with the court’s findings. At its core, the foregoing analysis echoes the remark made by three judges of the PCIJ in 1927 that “the view that advisory opinions are not binding is more theoretical than real”.Footnote 177 Looking at the overall picture, we can better understand why the UK has gone from openly defying the ICJ, and much of the international community, to concluding a negotiated settlement with Mauritius over the Chagos Archipelago. The lessons learnt from this case could also inspire (legal) action in other pressing contexts.
It seems that with the UK-Mauritius Agreement the illegality of the UK’s presence in the Archipelago will finally come to an end, its administration of Diego Garcia taking place in the future with Mauritius’s consent. Mauritius’s decolonisation may finally be complete. Despite this positive development, the Agreement as mentioned in Section II has been criticised for failing to respect Chagossian rights, including their right to return to their land and to be consulted in the negotiation process.Footnote 178 Indeed, neither Mauritius nor the UK recognise the Chagossians as an indigenous people,Footnote 179 despite UN and other experts’ assessment that they qualify as such and must be afforded all the rights attached to this status under international law.Footnote 180 Such rights include, in particular, the right to participate in the decision-making process concerning their land and the right to return, while reparations must also be made for the violation of their right to self-determination.Footnote 181 During the negotiations, the two states insisted that this was a matter for the two Governments (although, seemingly, both the US and India have been involved in the process), but that they have sought to engage with and keep “informed” the Chagossian communities.Footnote 182 At face value, this approach falls short of the duty to consult the Chagossians, while the final text of the Agreement guarantees neither their right to return nor the provision of reparations to them, with resettlement not being permitted anyway in Diego Garcia.Footnote 183
In the Chagos advisory opinion, the ICJ did not directly address the question of reparations and, as regards the right to return, the court stated that “[as] regards the resettlement […] this is an issue relating to the protection of human rights of those concerned, and should be addressed by the General Assembly during the completion of the decolonization of Mauritius”.Footnote 184 At the time, Judges Cançado Trindade and Salam criticised this omission, considering the duty to provide reparations and the right to return as consequences stemming from the UK’s wrongful conduct.Footnote 185 UN General Assembly Resolution 73/295, in turn, “urge[d]” the UK “to cooperate with Mauritius in facilitating the resettlement” and to “pose no impediment or obstacle” to it, but made no explicit reference to the Chagossians’ right to return. To an extent, these are reflections of the persistent state-centric outlook of international law, especially in institutional settings such as the UN General Assembly and the ICJ. Attending to individual and collective rights is, however, crucial in redressing the wrongs of the past and ensuring long-term compliance with international law.
While the UK will likely ratify the Agreement,Footnote 186 where individual rights are ignored, domestic courts have proven powerful partners to individuals in impeding the executive’s ability to act in accordance with certain international obligations that prioritise state interests over individual rights. The long-standing immunity dispute between Germany and Italy provides ample proof.Footnote 187 The normative effects of ICJ advisory opinions as analysed throughout this article could find expression in this context, too.
Open access
