US President Donald Trump has accused the UK of “making a big mistake” over its decision to return sovereignty of the Chagos Islands to Mauritius.

The plan would allow the UK and US to lease back a joint UK-US military base on the largest island Diego Garcia.

In a post on Truth Social, Trump said: “Our relationship with the United Kingdom is a strong and powerful one, and it has been for many years, but Prime Minister Starmer is losing control of this important Island by claims of entities never known of before.”

The post was made just a day after the US State Department said it supported the decision.

The US has been pushing Iran to agree to curb its nuclear programme.

In light of this, Trump suggested that losing control of Diego Garcia, the largest of the Chagos Islands, would be a bad move for national and international security.

“Should Iran decide not to make a Deal, it may be necessary for the United States to use Diego Garcia, and the Airfield located in Fairford, in order to eradicate a potential attack by a highly unstable and dangerous Regime — An attack that would potentially be made on the United Kingdom, as well as other friendly Countries. Prime Minister Starmer should not lose control, for any reason, of Diego Garcia, by entering a tenuous, at best, 100 Year Lease,” Trump wrote on Truth Social.

ITV News asked the Foreign Secretary whether Sir Keir Starmer has not given permission for President Trump to allow US aircraft to use British bases, including Diego Garcia, to support a potential attack on Iran.

Speaking exclusively to ITV News, Yvette Cooper said she supports “the US talks taking place with Iran”.

She added she believes increasing “diplomatic and economic pressure” on Iran was the “right thing to do”.

Trump previously critiqued the Chagos deal, and opposition from the US president was one of the reasons the British government delayed its Chagos Island bill in January.

However, at the start of February, he described the deal as “the best” call Starmer could make.

Now, the US President appears to have a different opinion.

But what is the deal, and why is it so controversial? ITV News explains.

What are the Chagos Islands?

The Chagos Islands are an archipelago in the Indian Ocean, approximately 1,250 miles north-east of Mauritius.

The largest of the islands, Diego Garcia, is home to a joint UK-US military base.

In 1814, the islands became classed as a British territory, while still being governed by Mauritius.

Why are the Chagos Islands important?

The remote chain of more than 60 islands is located in the middle of the Indian Ocean off the tip of India, south of the Maldives. But they are hugely strategically important.

The UK-US military base on Diego Garcia has supported US military operations from Vietnam to Iraq and Afghanistan. In 2008, the US acknowledged it also had been used for clandestine rendition flights of terror suspects.

More recently, the US deployed several nuclear-capable B-2 Spirit bombers to Diego Garcia amid an intense airstrike campaign targeting Yemen’s Houthi rebels.

The US has described the base, which is home to about 2,500 mostly American personnel, as an “all but indispensable platform” for security operations in the Middle East, South Asia and East Africa.

Why is the UK returning them to Mauritius?

In the 1980s, Mauritius claimed sovereignty over the islands.

In 2019, the International Court of Justice (ICJ) ruled in its favour, saying the UK was “under an obligation to bring to an end its administration of the archipelago as rapidly as possible”.

The Conservative government, under Rishi Sunak, began the negotiations to return the Chagos Islands, and Starmer’s Labour government finalised the deal, announcing it in May 2025.

Starmer said the deal was necessary, because the UK would “not have a realistic prospect of success” of winning a legal fight against Mauritius, given the ICJ’s ruling.

Failure to secure a deal threatened the legitimacy of the Diego Garcia base and ran the risk of hostile states like China establishing military bases on neighbouring islands.

Under the agreed deal, a 24-nautical-mile buffer zone would be put in place around the island where nothing can be built or placed without UK consent.

What terms were agreed in the deal?

On October 3, the UK announced a deal which would see the sovereignty of the remote Chagos Islands handed to Mauritius.

That deal also includes Diego Garcia.

Under the terms of the deal, the UK would cede sovereignty of the islands to Mauritius, making it the first time in 50 years that the status of the islands would be undisputed and legally secure.

In return, the UK would be allowed to lease the island of Diego Garcia to maintain the military base for a period of 99 years.

This would be done for £101 million per year, or £3.4 billion overall when adjusted for inflation, according to Whitehall.

What was said about the deal?

Despite the process having begun under Sunak’s Conservative government, opposition parties strongly objected to the move.

Critics of proposals to hand over the islands to Mauritius fear the move will benefit China, which has a growing reach in the region.

The Conservatives claimed Sir Keir Starmer had offended the Chagossian people when he said opponents of his deal to hand over the Chagos Islands were akin to Russia, China and Iran.

Shadow Foreign Secretary Dame Priti Patel said: “Starmer has slandered the Chagossian community, and he is so arrogant and out of touch with British values and the national interest that he has resorted to baseless and disgusting slurs – whilst he himself hands control of Chagos to a country that is actually cosying up with Russia and China.

Reform UK’s Nigel Farage has also voiced repeated opposition to the deal, arguing it risks national security.

The deal was also subject to a last-minute legal hurdle, when hours before it was due to be signed, a legal challenge prevented its completion, with Mr Justice Goose granting an injunction against the FCDO for “interim relief”.

The legal challenge had been brought against the Foreign, Commonwealth and Development Office (FCDO) by two British women born on the Chagos Islands, but was dismissed in the High Court.

An estimated 10,000 displaced Chagossians and their descendants now live primarily in Britain, Mauritius and the Seychelles. Many of them want to return to the islands, and some have fought unsuccessfully in UK courts for many years for the right to go home.

Chagossians say they were left out of the political negotiations, which have left them unclear on whether they and their descendants could ever be allowed to return to their homeland.

Does the US agree?

Trump initially seemed to oppose the deal, with Nigel Farage frequently referencing this in Commons’ debates.

Despite this, Trump signed the agreement last year, with US Secretary of State Marco Rubio saying it “secures the long-term, stable and effective operation of the joint US-UK military facility at Diego Garcia, which is critical to regional and global security”.

The deal looked to be in jeopardy again in January, after Trump took to his social media platform Truth Social to attack the agreement, linking it to his ambitions to acquire Greenland.

In a direct attack on the UK, the president called the Chagos decision “another in a very long line of National Security reasons why Greenland has to be acquired”.

Trump wrote: “Shockingly, our ‘brilliant’ NATO Ally, the United Kingdom, is currently planning to give away the Island of Diego Garcia, the site of a vital US Military Base, to Mauritius, and to do so FOR NO REASON WHATSOEVER.”

In response, the government defended the deal, which it said had been welcomed by key allies, including the US.

A government spokesperson said in a statement: “The UK will never compromise on our national security. We acted because the base on Diego Garcia was under threat after court decisions undermined our position and would have prevented it from operating as intended in the future.”

In February, after what Trump described as “very productive discussions with Prime Minister Keir Starmer about the Island of Diego Garcia”, the US President appeared to support the deal.

Now, Trump has said Sir Keir Starmer is “making a big mistake”, citing national and international security concerns in his Truth Social post.