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International Law and the Chagos Archipelago

The British Indian Ocean Territory (BIOT), or the Chagos Archipelago, is one of 14 UK Overseas Territories. On 22 May 2025, the governments of the UK and Mauritius signed a treaty to recognise Mauritian sovereignty over the archipelago. The treaty has not yet come into force, and ratification by the UK has been paused until an agreement can be reached between the US and the UK over the future of the US base on the island of Diego Garcia.

This briefing recaps the main international legal issues leading to the agreement on Mauritian sovereignty over the Chagos Archipelago, and its subsequent stalling. It also explains some of the legal risks that were considered by the UK Government in its explanatory memorandum on the UK–Mauritius Treaty in 2025. Further subsequent legal issues, including challenges to the UK–Mauritius agreement itself, are not covered in this briefing but are set out in the further reading resources at the end of this paper.

The international legal issues

There have been three key legal cases that have impacted on the UK’s administration of the BIOT:

1. At the Permanent Court of Arbitration, Chagos Marine Protected Area Arbitration (Mauritius v. United Kingdom), 18 March 2015
2. At the International Court of Justice (ICJ), its 2019 Advisory Opinion: Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965
3. Proceedings of a dispute between Mauritius and the Maldives regarding delimitation of their maritime boundary at a Special Chamber of the International Tribunal for the Law of the Sea (ITLOS), 2019-2023]

The 2015 Permanent Court of Arbitration case was a challenge by Mauritius against the UK’s attempt to establish a Marine Protected Area (areas established in a particular area of the sea to protect the environment, prevent pollution, or protect marine life). This case centred around the United Nations Convention on the Law of the Sea and ultimately decided that the UK’s declaration of the MPA was unlawful, because it failed to take into account the rights of Mauritius. In particular, the tribunal decided that the UK’s agreements with Mauritius at Lancaster House in 1965 were legally binding and should have been taken into account.

On the other hand, the International Court of Justice went further in its 2019 Advisory Opinion. The ICJ was asked by the UN General Assembly to give its view on whether the decolonisation of Mauritius was completed lawfully:

Was the process of decolonization of Mauritius lawfully completed when Mauritius was granted independence in 1968, following the separation of the Chagos Archipelago from Mauritius and having regard to international law

The specific legal rules considered by the court were those relating to the right of self-determination of peoples, and how they apply during any process of decolonisation. The ICJ advised that the separation of the Chagos Archipelago from Mauritius was not “based on the freely expressed and genuine will of the people of the territory concerned”. This meant that “the decolonization of Mauritius was not conducted in a manner consistent with the right of peoples to self-determination”.

The ICJ went on to advise that “the United Kingdom’s continued administration of the Chagos Archipelago constitutes a wrongful act entailing the international responsibility of that State” and said that the UK’s administration of the islands must be brought to an end.

Finally, following the ICJ’s Advisory Opinion, a dispute between Mauritius and the Maldives regarding delimitation of their maritime boundary was transferred to a Special Chamber of the International Tribunal for the Law of the Sea (ITLOS) in September 2019. The UK was not a party to the case, but the tribunal ultimately decided that a maritime boundary could be established between Mauritius and the Maldives, with the Mauritian side covering the Chagos Archipelago while the Maldives maritime zones remain to the north.

2025 treaty

The 2025 agreement between the UK and Mauritius provides for Mauritius to exercise full sovereignty over the Chagos Archipelago. However, it gives the UK some rights over Diego Garcia (one of the islands of the archipelago) where there has been a joint UK and US military base since the early 1970s. The UK Government says this resolves the long-standing BIOT sovereignty dispute and secures the military base for continued operations in the long-term.

A bill to implement the treaty fell in 2026

The treaty requires legislation to be implemented in UK law. However, a bill to do this, the Diego Garcia Military Base and British Indian Ocean Territory Bill, was not passed before prorogation of the 2025-2026 session of Parliament and so fell.

In addition to legislation, the treaty requires the UK and US to update or replace their existing legal “exchange of notes” on the US use of the island, putting their existing arrangements around the base on Diego Garcia on a new legal footing. The government explained to the Commons on 13 April 2026 that the UK “could not let the treaty enter into force without US support”, and that a change US President Trump’s opinion on the matter meant that it:

… has become impossible to agree at political level an update to the 1966 UK-US agreement concerning the availability of defence purposes of the British Indian Ocean Territory, known as the exchange of notes, which is necessary to ratify the treaty.

Negotiations are continuing in 2026

In September 2026, President Trump, following a meeting with Prime Minister Andy Burnham at the UN General Assembly, said that the US currently does not support the agreement. Prime Minister Burnham said he would work with the US and Mauritius to find a solution. Plans for annual payments to Mauritius for the lease of the land for the Diego Garcia military base have been put on hold.

Further reading

Background on the context, agreement, and details of the 2025-26 bill, are outlined in the following Commons Library Briefings:

• Disputes over the British Indian Ocean Territory: February 2021 update, 8 February 2021
• 2025 treaty on the British Indian Ocean Territory/Chagos Archipelago, 8 September 2025
• Diego Garcia Military Base and British Indian Ocean Territory Bill 2024-25, 13 October 2025
• Diego Garcia Military Base and British Indian Ocean Territory Bill: Consideration of Lords amendments, 19 January 2026