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VIDEO EXPLAINER | 1 NOVEMBER 2026
On 22 May 2025 the United Kingdom and Mauritius signed a treaty under which Mauritius gets sovereignty over the whole Chagos Archipelago while the UK leases back the Diego Garcia military base for at least 99 years at about 101 million pounds a year. The deal follows the International Court of Justice’s 2019 advisory opinion that the 1965 detachment of Chagos left Mauritius’s decolonisation incomplete. Through 2026 the treaty was still NOT in force: its implementing Bill stalled in the UK Parliament after US objections, Mauritius began exploring legal action over the delay, and in April 2026 the Bill died altogether — the United States would not clear the updated exchange of notes, and the Bill fell when Parliament was prorogued on 29 April 2026.
What happened
The UK first announced in October 2024 that it would cede sovereignty over the Chagos Islands to Mauritius, calling it a ‘historic political agreement’.
The United States, which operates the Diego Garcia base jointly with the UK, publicly supported the UK-Mauritius agreement.
The treaty was laid before both Houses of the UK Parliament on 22 May 2025 and was marked ‘not in force’ at that time.
Mauritius is free to organise the resettlement of Chagossians on every island of the archipelago except Diego Garcia.
India’s Ministry of External Affairs welcomed the treaty on 23 May 2025 as ‘a milestone achievement and a positive development for the region’.
India framed its support as consistency with its stand on decolonisation and respect for the sovereignty and territorial integrity of nations.
A UK High Court judgment rejected legal challenges brought by some Chagossians against the deal, holding several claims ‘unarguable’.
On 18 February 2026 President Trump said UK Prime Minister Keir Starmer was ‘making a big mistake’ over the Chagos deal and posted ‘DO NOT GIVE AWAY DIEGO GARCIA!’, saying the base might be needed against Iran.
Mauritius argues the delay is costing it money, because it cannot budget for the lease payments it has been promised.
On 13 April 2026 FCDO Minister of State Stephen Doughty told the Commons that, because the United States would not clear the updated exchange of notes, the Diego Garcia Bill could not complete its passage in that session and could not be carried over.
Parliament was prorogued on 29 April 2026 and the Diego Garcia Military Base and British Indian Ocean Territory Bill fell without completing its passage; it had not been reintroduced by September 2026.
Background and the rules behind it
The 2019 opinion was adopted by 13 votes to 1, the sole dissent being the American judge.
Britain detached the Chagos Archipelago from Mauritius on 8 November 1965 by Order in Council and created the British Indian Ocean Territory (BIOT).
In 2015 the Permanent Court of Arbitration held that the Marine Protected Area the UK declared around Chagos in 2010 was created in violation of international law.
Mauritius became independent on 12 March 1968 and a republic on 12 March 1992; its capital is Port Louis.
Under the Treaty of Paris of 1814 France ceded Isle de France (Mauritius) and the Chagos Islands to the United Kingdom; the French had started coconut plantations there in the 1770s with enslaved labour.
The Chagos Archipelago is a group of seven atolls with more than 60 islands, about 500 km south of the Maldives; in 1903 it was administratively separated from the Seychelles and attached to Mauritius.
Key terms
- Advisory opinion: A non-binding legal opinion the ICJ gives when an authorised UN organ or agency asks it a legal question.
- Decolonisation: The process by which a colony achieves self-government, which international law says must respect the territory’s integrity.
- Self-determination: The right of a people to freely decide its political status, treated by the ICJ as customary international law.
- British Indian Ocean Territory (BIOT): The British Overseas Territory created in 1965 out of the Chagos islands detached from Mauritius.
- Leaseback: An arrangement in which a seller or ceding state keeps use of the property it has transferred, on lease.
- Buffer zone: A belt around the base, here 24 nautical miles, inside which nothing may be built without the UK’s consent.
Constitutional / Legal / Institutional framework
- ICJ advisory opinion, Legal Consequences of the Separation of the Chagos Archipelago from Mauritius in 1965 (25 February 2019): Decolonisation of Mauritius was not lawfully completed in 1968; the UK must end its administration of Chagos as rapidly as possible; its continued administration is an internationally wrongful act.
- UN Charter, Article 96: The General Assembly and Security Council may ask the ICJ for an advisory opinion on any legal question; other organs and agencies may do so if authorised.
- UNGA resolution 71/292 (22 June 2017): Referred the Chagos question to the ICJ for an advisory opinion.
- UNGA resolution 73/295 (22 May 2019): Demanded that the UK withdraw its colonial administration from the Chagos Archipelago unconditionally within six months.
- UK-Mauritius Agreement concerning the Chagos Archipelago including Diego Garcia (22 May 2025): Mauritius gains sovereignty; the UK leases Diego Garcia for 99 years, extendable by 40, with annual payments and a 24-nautical-mile buffer zone.
- Diego Garcia Military Base and British Indian Ocean Territory Bill (UK): The domestic legislation needed to give effect to the treaty; stalled from January 2026 and then lost altogether — it could not be carried over and fell at the prorogation of 29 April 2026, so the treaty is signed but not in force.
CLAT angle
- A decolonisation passage that turns on the difference between an ICJ advisory opinion (non-binding but weighty) and a contentious judgment (binding on the parties).
- Direct question on which organ asked the ICJ for the opinion and under which resolution — GA resolution 71/292, not the Security Council.
- Legal-reasoning set on a principle like ‘a colonial territory may not be dismembered before independence’: apply it to the 1965 detachment of Chagos.
- Statement question on the treaty terms: sovereignty to Mauritius, 99-year lease of Diego Garcia, 40-year renewal, 24-nautical-mile buffer — with the trap that the base was NOT handed over.
- Signature vs ratification: a treaty signed in May 2025 but not in force in 2026 is a clean test of when a treaty binds.
Key facts
| Fact | Detail |
|---|---|
| 1814 | France cedes the Chagos islands to Britain; they are administered from Mauritius |
| 1965-11-08 | Britain detaches Chagos from Mauritius and creates the British Indian Ocean Territory |
| 1967 | Forced removal of the Chagossians begins; about 2,000 are deported by 1973 |
| 2019-02-25 | ICJ advisory opinion: decolonisation of Mauritius was not lawfully completed |
| 2025-05-22 | UK-Mauritius treaty signed; Diego Garcia leased back for 99 years |
| 2025-05-23 | India welcomes the treaty as a milestone for decolonisation |
| 2026-02-18 | Trump says Starmer is ‘making a big mistake’ and posts ‘DO NOT GIVE AWAY DIEGO GARCIA!’ |
| 2026-04-29 | Parliament is prorogued and the Diego Garcia Bill falls |
Mnemonic / memory hook
Remember the trap.
Three traps. (1) The UK did NOT give up the base — sovereignty goes to Mauritius, Diego Garcia is leased back for 99 years. (2) The ICJ gave an ADVISORY OPINION (2019), not a judgment ordering return; it is not binding, which is why a treaty was still needed. (3) The treaty is signed but, through 2026, not ratified and not in force — never write that Mauritius ‘got back’ the islands in 2025.
Sources
Primary documents and reporting listed in the fact base: lordslibrary.parliament.uk, newsonair.gov.in, state.gov, ncl.ac.uk, gov.uk, aljazeera.com, hansard-api.parliament.uk, bills-api.parliament.uk. Lead image: Photo: Diliff, CC BY-SA 2.5, via Wikimedia Commons.
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