What are the Chagos Islands – and why is the UK returning them to Mauritius?

Original article by Daniel Boffey Chief reporter

In a post on Truth Social, Donald Trump has described the UK’s plans to hand over the Chagos Islands to Mauritius as “an act of great stupidity”. He claimed that it had made it all the more important for him to take Greenland from Denmark. Kemi Badenoch and Nigel Farage, while critical of Trump over his threats of tariffs on European countries who oppose his territorial land grab, have been quick to lend Conservative and Reform support to the US president in his criticism of the UK-Mauritius treaty, which is now making its way through parliament.

What are the Chagos Islands?

They are an archipelago in the Indian Ocean, located about 5,800 miles south-east of the UK and 1,250 miles north-east of Mauritius. They became British territory along with Mauritius in 1814 as part of the treaty of Paris after the defeat of Napoleon. The group of islands were designated as British Indian Ocean Territory in 1965 and detached from Mauritius, which became independent in 1968. The deal then was that the islands would return to Mauritius once they were no longer needed for Britain’s defence. A joint UK-US military base was built on the largest of the islands, Diego Garcia. Displaced residents were resettled in Mauritius and Seychelles, while some were relocated to the UK in 2002.

Why are they being returned to Mauritius now?

Since the 1980s, Mauritius has claimed sovereignty over the islands and pursued its case through international courts and tribunals. In 2019, the international court of justice (ICJ), the principal judicial organ of the United Nations, ruled in a non-binding judgment that the process of decolonisation of Mauritius was not lawfully completed when it was granted independence in 1968 and that the UK was “under an obligation to bring to an end its administration of the archipelago as rapidly as possible”.

What has happened since that ruling?

Under the government led by Rishi Sunak, a Conservative prime minister, it was announced in 2022 that the UK and Mauritius had agreed to begin negotiations on the exercise of sovereignty over the Chagos archipelago. It was argued that Britain’s defence was best served by having legal certainty over the Chagos Islands due to the risk of continued challenges in international courts. It was an acknowledgment that the UK respected international law.

The UK government stressed that agreement would “ensure the continued effective operation of the joint UK-US military base on Diego Garcia”. The Conservative government held 11 rounds of negotiations with the Mauritians before the general election in July 2024. After Labour won the summer election, the two governments published a joint statement on 3 October 2024 announcing that a political agreement had been reached. The treaty was formally approved and signed by Keir Starmer and his counterpart, the Mauritian prime minister, Navin Ramgoolam, on 22 May 2025. Starmer said the deal was vital as the UK would “not have a realistic prospect of success” if Mauritius pursued legal action. He explained that the UK could have faced a provisional measures order “within a matter of weeks”.

The UK could just ignore such orders but without an agreement it would not have legal grounds to prevent China or other nations from establishing bases on the outer islands or conducting joint exercises near Diego Garcia, it was claimed.

What was the political reaction in Britain and the US?

In response to the deal, the US government issued a statement in which is said that after a “comprehensive interagency review”, it had determined that the agreement “secures the long-term, stable, and effective operation of the joint US-UK military facility at Diego Garcia”.

During a meeting with the British prime minister in the Oval Office in February 2025, Trump said he was “inclined” to go along with the agreement and that he had “a feeling that it’s going to work out very well”. The Conservatives under the leadership of Badenoch criticised the deal, describing it as a “surrender” and arguing that the UK was “handing over British territory and paying upwards of £30bn to do so” to Mauritius, a country she claimed “openly cuddles up to China and Russia”. The shadow defence secretary, James Cartlidge, claimed the Conservatives had not signed an agreement while in government because they “fundamentally oppose the idea of spending billions of pounds on a surrender tax to lease back land that we currently own freehold”.

What are the terms of the deal?

Mauritius takes sovereignty of the islands but the UK maintains an initial 99-year lease of Diego Garcia with an option of extension. The UK will pay for that lease. The average annual cost during the initial 99-year period of the treaty is estimated by Whitehall to be £101m in 2025-26 prices. The total projected cost of the finance package using a net present value methodology is £3.4bn, although some argue it could be higher.

Why has Donald Trump changed position?

It may just be a coincidence but Starmer held a press conference on Monday – hours before the US president’s post on Truth Social – in which he criticised Trump for threatening to impose tariffs on the imports of European countries who had opposed his claim on Greenland, which is part of the kingdom of Denmark. The prime minister told reporters that “threatening tariffs on allies is the wrong thing to do, completely wrong”. He played down any suggestion of retaliatory action, something that is being actively considered by the EU. Starmer had followed up with a phone call to the capricious president. It appears not to have gone well.