Summary of Chagos Archipelago Case and the Use of Right to Self-Determination

Introduction

This paper will review the unlawful decolonisation process concerning the 2019 ICJ Judgement on the Legal Consequence of the Separation of the Chagos Archipelago from Mauritius in 1965. The case study observes that the 1965 agreement did not comply with the territorial integrity of the Chagos Archipelago, a non-self-governing state, whose right to self-determination was not practiced in the decolonisation process of Mauritius. Furthermore, this paper will mention the significance of the ICJ’s Advisory Opinion as a global legal tool to highlight the right to self-determination for former colonial states at the UN General Assembly.

The Petition

Initially, on 20th December 2010, the Republic of Mauritius filed a case at the Permanent Court of Arbitration against the United Kingdom of Great Britain and Northern Ireland for attempting to establish a Marine Protected Area (MPA) around the Chagos Archipelago. The case discovered the fishery rights of Mauritian vessels were breached and Chagossians were forcibly displaced for an MPA and therefore, ruled in violation of the United Nations Convention on the Law of the Sea (UNCLOS)[1]. The Court questioned the legality of British Occupation and announced Mauritius entitlement to the Chagos Archipelago in 2015. However, the PCA case only covered the aspect of an MPA establishment and the UK’s failure to consult Mauritius, which resulted in Mauritius bringing the case of the Chagos Island forward at the UN General Assembly in the summer of 2017 and, later, the UNGA requested for International Court of Justice’s Advisory Opinion[2].

Background

The case is drawn from the decolonisation process of Mauritius by the British government during the 1960s. In 1965, the Chagos Archipelago, administrated under the colony of Mauritius, was declared as a British Indian Ocean Territory (BIOT) before Mauritius fully gained independence. Between 1967 and 1973, the population of the Chagos Islands was displaced, in pursuit of the UK-US military base in Diego Garcia, the largest Chagos Archipelago Island[3]. This indicates the current involvement of the American government in the decision-making process for finalising negotiations. In 1980, the Organisation of African Unity (OAU) embraced resolution 99 (XVII), which demanded the unconditional return of Diego Garcia to Mauritius[4]. Later that year, the Mauritian Prime Minister appealed for the BIOT to be disbanded to preserve the national heritage of Mauritius[5]. The OAU found that since decolonisation meant an unconditional transfer of power, the creation of the Chagos Archipelago as a BIOT violates international law due to the event taking place before the independence of Mauritius. 

In 2019, the ICJ ordered the British government to complete the decolonisation process and return the Chagos Archipelago to the Republic of Mauritius[6], to which the UK government failed to comply. In 2022, the UK government agreed to negotiate with Mauritius over the land dispute, which currently awaits to be signed[7].

Overview of the proceedings

Despite the last joint statement by Mauritius and the United Kingdom in January 2025, the case is yet to be concluded by a signed agreement[8]. This makes the ICJ case 8 years old, from the UN General Assembly’s resolution to request an advisory opinion in 2017 to the pending reparations. The International Court of Justice’s objective for the case was twofold by the questions posed by the UN General Assembly, first, to examine the lawfulness of the decolonisation process set for Mauritius in 1968 and the separation of Chagos Archipelago in 1965[9]. The primary purpose of the case introspected upon the international law obligations placed in the General Assembly, such as the Declaration on the Granting of Independence to Colonial Countries and Peoples of 1960 (resolution 1514 (XV)) which categorised the principle of self-determination and the transfer of power to people of non-self-governing states[10]. Further on, the case revisited the 2066 (XX) resolution and the 2232 (XXI) resolution of the General Assembly, which questions the territorial integrity of Mauritius by underlining the detachment of the islands for military installations[11]. Secondly, the court was required to discuss the implication of continued British administration of the Chagos and the inability of Mauritius to ensure the resettlement of Chagossians in the native region[12]. Additionally, since the court was requested for an advisory opinion to the UNGA, the jurisdiction and the implementation of Article 65, paragraph 1 of ICJ statute was also introspected.

As the case centres on the right to self-determination for a formerly British colony, illustrating erga omnes, providing the court authority for an advisory opinion. Upon noting the 1514 (XV) resolution which demands the unconditional and speedy end to all forms of colonial practice, the ICJ denotes the legal flaw in the 1966 resolution between Mauritius and the United Kingdom for settling military units in Diego Garcia in return for compensation, since it is inconsistent with self-determination and territorial integrity. Moreover, it was recognised that the detachment process was not based on the genuine will of the people and that it was agreed on the return of Chagos at a later date[13]. Regarding the second question, the court rendered the continued presence of the British administration as a wrongful act entailing international responsibility of that state[14].

Finally, after establishing its jurisprudence over the matter, the ICJ delivered its advisory opinion on the case, declaring that Mauritius’s decolonisation process was not lawfully completed and noting the United Kingdom’s obligation to end its administration in the Chagos Archipelago as rapidly as possible[15].

Position of the parties

The claimant’s argument briefly outlined that the process of detachment was carried out in haste and with considerable pressure on Mauritian officials[16]. Furthermore, the claimant expressed bad and unfair treatment towards the Chagossians who were removed forcefully from the islands. On the other hand, the respondent argued that the detachment of the Chagos Islands and granting independence to Mauritius were two separate issues. The respondent counterargued that the Chagos Islands were not integral to the colony of Mauritius, therefore attempting to redundant the application of territorial integrity from paragraph 6 of resolution 1514 (XV)[17]. Additionally, the British government protested against Chagos separation being a condition for Mauritian independence. The United Kingdom disapproved of Mauritius’s allegation of the duress of the detachment process by claiming it to be rather a complex internal political situation within the Mauritius colony[18].

Moreover, based on the dispute being bilateral, the British government argued for the court to not deliver an Advisory Opinion, on the claims it would breach the non-circumvent rule. The United Kingdom associated the case with political concepts and dismissed the matters of the dispute to be legal affairs of decolonisation and self-determination. The reason for this underlined in the aspect brought by the UK that the elected representatives did accept the agreement finalised at the Lancaster House, henceforth completing the act of decolonisation.

Discussion

         The case centred around the question of decolonisation and self-determination, and whether the dispute was contested by international jurisdiction. Both parties of the case had polar opposite claims on the international legality of the case. There were four key points of contention addressed in the case. Firstly, the applicability of self-determination during the period of the detachment of Chagos, and if, then was the principle of self-determination violated? Resolution 1514 (XV) which associates complete liberation of a colonial state and devotes the interests of dependent persons was adopted in 1960[19], five years before the detachment agreement was signed. As the United Kingdom did argue that Mauritian representatives had shown consent, it is essential to observe that Mauritius was still at the time a colony and presented with political pressure from the House. The UNGA appeal refers to a report by Sir Oliver Wright which mentions Prime Minister Harold Wilson informing Mauritius Premier Ramgoolam, illustrating the pressure from the Lancaster House:

“On the Defence point, Diego Garcia could either be detached by order in Council or with the agreement of the Premier and his colleagues”[20].

         The 1960 resolution and Wright’s report answered and countered the United Kingdom’s argument by showing the applicableness of self-determination in Mauritius decolonisation and the violation of the principle. Secondly, the sovereignty over the Chagos Archipelago, and was Chagos territorially integral to Mauritius? Resolution 2066 (XX) on the “Question of Mauritius” analysed that establishing a military base in Chagos would violate the territorial integrity of Mauritius since Chagos is historically joined with Mauritius[21]. Resolution 2232 (XXI) reiterated that the establishment of military bases in former colonial territories would be incompatible with resolution 1514 (XV)[22]. These resolutions showcased that Chagos is integral to Mauritius sovereignty, and therefore, the Republic of Mauritius has the rightful sovereignty over the Chagos Archipelago and not the United Kingdom.

         Thirdly, the forced removal of Chagossians from their nativity. The Human Rights Watch submitted a report in 2023 that viewed how the United Kingdom had removed Chagossian and described the act as a colonial crime. The BIOT authorities transferred Chagossians to either Mauritius, Seychelles, or the United Kingdom by British citizenship, and refused their right to return[23]. The report refers that the ICJ announced the compensation for the removal as unfair and stated Chagossian’s right to resettle. Lastly, the legal obligations that the United Kingdom would be required to undertake if it is proved that there was a violation of international law principles. As the international law statutes prove the UK’s violations, the UK had the following legal obligations to complete the decolonisation process by transferring power to Mauritius, preserving the right to resettle for Chagossians, and dismantling all BIOT operations in the region[24]. However, as the military base is shared with the United States in Diego Garcia, their involvement to halt operations would be required as well, and therefore, after the inauguration of President Trump at The White House, the Downing Street was waiting for the new Trump administration’s output on the ongoing agreement[25]. Presently Keir Starmer provides clarity that the Chagos deal needs to be finalised with discussions solely between Mauritius and the U.K. since Trump approves of the agreement on Chagos and will stand with the United Kingdom[26].

Judgement

The court recognised its use of advisory opinion permissible to the organ requested, i.e., the General Assembly. The ICJ observed international law’s applicability between the periods of this case 1965 and 1968, and further accepted jurisdiction on the case, while the United Kingdom vehemently opposed an advisory opinion[27]. The court moved forward and discovered that the heavy dossier and written statements presented assisted in settling complex and disputed issues on self-determination[28]. Further, the court dismissed the non-circumvent principle and the notion of the court dealing with a bilateral case since the case centred around international law surrounding decolonisation[29].

            With this, another question posed to the court centred on the appropriateness for the court to reexamine a question allegedly held by the arbitral tribunal. Since the court clarified that the Arbitral tribunal focused on Marine Protected Areas (MPA) and is not the same as the case presented to the court, the court found that the principle of res judicata does not prevent it from providing an advisory opinion[30]. Therefore, it dismissed the grounds of res judicata to not provide an advisory opinion. As the court did not discover a reason that prevented it from providing an advisory opinion, the court settled on its judgement of 2019. The opinion reads that the decolonisation process of Mauritius and Chagos Archipelago was uncompleted and unlawful and, that the United Kingdom has a legal obligation to lawfully complete the process[31]. While the court asked for a speedy transition from the United Kingdom, and later, the UNGA presented the respondent with a 6-month deadline to undertake action[32], the United Kingdom refused to support the findings of the judgement and continued to maintain its proclaimed sovereignty over Chagos until 2022.

Conclusion 

            The court concluded in favour of Mauritius claims over the Chagos Island. The case did assist in supporting the territorial integrity of the Republic of Mauritius and the rights of Chagossians to return to their nativity at an international level. The UN General Assembly passed a resolution in the same year of the judgment granting 6 months to the United Kingdom to return the island, which the United Kingdom disregarded. Even though now the negotiations are in the process of being finalised, the case did signify the right to self-determination of a small state, previously a British colony. The United Kingdom’s initial disregard of the judgement signifies that ICJ advisory opinion have no binding force and simply are legal advice. In summary, the court attempted to help rectify the decolonial process error and reinforce international law through resolution 1514 (XV). Ultimately, the case also showcases the use of the court’s judgement at the General Assembly to urge the United Kingdom to follow through.

Bibliography:

Legal Consequence of the Separation of the Chagos Archipelago from Mauritius in 1965. (Mauritius v The United Kingdom), Judgment I.C.J. Reports 2019

Legal Consequence of the Separation of the Chagos Archipelago from Mauritius in 1965. (Mauritius v The United Kingdom), Written Statement of the Republic of Mauritius I.C.J. Reports 2018

Legal Consequence of the Separation of the Chagos Archipelago from Mauritius in 1965. (Mauritius v The United Kingdom), Written Statement of the United Kingdom of Great Britain and Northern Ireland I.C.J. Reports 2018

Chagos Marine Protected Area Arbitration (Mauritius v United Kingdom). Cases, Permanent Court of Arbitration 2025. https://pca-cpa.org/en/cases/11/ 

Chagos Marine Protected Area Arbitration (Mauritius v United Kingdom). Notice of Arbitration. Cases, Permanent Court of Arbitration 2010. https://pcacases.com/web/sendAttach/1791 

Human Rights Watch, (2023). “That’s When the Nightmare Started: US and UK Forced Displacement of the Chagossians and Ongoing Colonial Crimes” https://www.hrw.org/report/2023/02/15/thats-when-nightmare-started/uk-and-us-forced-displacement-chagossians-and#6146 

Mills, Claire and Butchard, Patrick. (2021). “Disputes over the British Indian Ocean Territory: February 2021 Update” House of Commons Library, UK Parliament https://commonslibrary.parliament.uk/research-briefings/cbp-9134/ 

Foreign, Commonwealth & Development Office. (2025) “UK-Mauritius joint statement on Chagos Archipelago, 13 January 2025.” GOV.UK

Siddique, Harron. (2024) “A timeline of the UK’s history with the Chagos Islands”.  the Guardian. https://www.theguardian.com/world/2024/oct/03/timeline-uk-history-chagos-islands-mauritius 

Crerar, Pippa and Walker, Peter. (2025) “UK stalls Chagos Islands deal until Trump administration can ‘consider detail’”. The Guardian. https://www.theguardian.com/world/2025/jan/15/uk-chagos-islands-handover-mauritius-donald-trump-diego-garcia 

United Nations. (2025). “United Nations and Decolonisation.” https://www.un.org/dppa/decolonization/en/about 

UNGA. (2019). Advisory opinion of the International Court of Justice on the legal consequences of the separation of the Chagos Archipelago from Mauritius in 1965. Seventy-third session, Agenda item 88.

UN Affairs. (2024) “Chagos Islands: “UK’s last African colony returned to Mauritius” UN News. https://news.un.org/en/story/2024/10/1155326 

Fisher, Lucy (2025). Donald Trump signs off Chagos Islands deal between UK and Mauritius. Financial Times. https://www.ft.com/content/e03ab89d-42fc-44ab-89fe-7ed086f0ab3a 

Footnotes:

[1] Notice of Arbitration, “2. The ‘MPA dispute” para 4, page 4. In The dispute concerning The ‘Marine Protected Area’ related to the Chagos Archipelago (Mauritius v. United Kingdom). (PCA, 2010).

[2] Siddique, 2024

[3] Summary, “Events leading to the adoption of the request for the advisory opinion” paras 25-53, page 2. In Legal Consequence of the Separation of the Chagos Archipelago from Mauritius in 1965. (ICJ, 2019)

[4] ibid, page 3

[5] ibid

[6] VI Operative Paragraph, paras 183, page 13 In Legal Consequence of the Separation of the Chagos Archipelago from Mauritius in 1965 (ICJ, 2019)

[7] UN Affairs, (2024)

[8] Foreign, Commonwealth & Development, (2025)

[9] I. History of the Proceedings, paras 1-24, page 1 (ICJ, 2019)

[10] II Events Leading to the Adoption of the Request for the Advisory Opinion, paras 25-53, page 2, (ICJ, 2019)

[11]ibid

[12] I. History of the Proceedings, paras 1-24, page 1 (ICJ, 2019)

[13] V. The questions put to the Court by the General Assembly, Part 1, Section B” paras 132-182, page 10. (ICJ, 2019)

[14] Ibid, Part 2, page 12. (ICJ, 2019)

[15] ibid, VI Operative Paragraph, paras 183, page 13

[16] “Chapter 3: The process of decolonisation and the detachment of the Chagos Archipelago from Mauritius, II The struggle to independence” para 3.13, page 62. In Written Statement of the Republic of Mauritius, vol 1. (ICJ, 2018)

[17] “Chapter IV: Response to question (a)The process of decolonisation was lawfully completed in 1968” para 4.49, page 81. In Written Statement of the United Kingdom of Great Britain and Northern Ireland. (ICJ, 2018)

[18]  ibid, para 4.10, page 60

[19] United Nations and decolonisation, 2025

[20] III, Part B, Para 107, page 28, In Advisory opinion of the International Court of Justice on the legal consequences of the separation of the Chagos Archipelago from Mauritius in 1965, (UNGA, 2019)

[21]  V, Part 1, Section C, paras 132-182, page 10. (ICJ, 2019)

[22] ibid

[23] Human Rights Watch, 2023

[24] V, Part 2, paras 132-182, page 12. (ICJ, 2019)

[25] Crerar and Walker, 2025

[26] Fisher, Lucy (2025) Donald Trump Signs off Chagos Islands Deal between UK and Mauritius, Financial Times https://www.ft.com/content/e03ab89d-42fc-44ab-89fe-7ed086f0ab3a.

[27] V, Part 1, Section a, paras 132-182, page 8. (ICJ, 2019)

[28] III, Part 1, paras 54-91, page 4. (ICJ, 2019)

[29] ibid, Part 4, page 5

[30] ibid, Part 3, page 4

[31] VI Operative Paragraph, paras 183, page 13. (ICJ, 2019)

[32] Mills and Butchard, 2021