Trump, who had endorsed the deal when it was signed in May last year, posted on social media Tuesday that the UK "giving away extremely important land is an act of GREAT STUPIDITY, and is another in a very long line of National Security reasons why Greenland has to be acquired".

Trump has threatened to slap tariffs on Britain and other European countries for opposing his claims on Greenland, an autonomous territory of Denmark.

The Chagos agreement will see Britain hand the archipelago -- some 2,000 kilometers northeast of Mauritius -- to its former colony and pay to lease the US-UK military base for a century.

Britain insisted the agreement, which is in the final stages of being scrutinized in parliament, would go ahead despite Trump's U-turn.

"Our position hasn't changed," a UK government spokesperson said, hailing the deal's "robust provisions" and highlighting that nations from Australia to Japan had welcomed it.

"This deal secures the operations of the joint US-UK base on Diego Garcia for generations," the spokesperson added, referring to the largest of the Chagos islands.

Mauritius said they had "taken note" of Trump's comments criticizing the deal, but said the issue was no longer up for debate.

"The sovereignty of the Republic of Mauritius over the Chagos archipelago is already recognized without ambiguity under international law and should no longer be a subject for debate," Mauritian Attorney General Gavin Glover said in a statement.

US Secretary of State Marco Rubio had said in May last year that Washington "welcomes the historic agreement".

"The Trump administration determined that this agreement secures the long-term, stable, and effective operation of the joint US-UK military facility at Diego Garcia," Rubio said in a statement at the time.

UK foreign office minister Stephen Doughty told parliament Tuesday that Britain would have "discussions" with the US in the coming days "to remind them of the strength of this deal and how it secures the base".

The UK kept control of the Chagos Islands after Mauritius gained independence from Britain in the 1960s.

But it evicted thousands of islanders, who have since mounted a series of legal claims for compensation in British courts.

In 2019, the International Court of Justice recommended that Britain hand the archipelago to Mauritius after decades of legal battles.

Challenges in international and domestic courts had left the status of the military base "under threat", according to the UK government.

The UK government spokesperson insisted the base was "one of the most significant contributions to the UK and US intelligence and security relationship".

"Almost every operation from the base is in partnership with the US," the spokesperson added.

The base played a major strategic role in the Cold War, offering proximity to Asia, and later served as a staging ground for US bombing campaigns in Afghanistan and Iraq.

It was used recently to launch B-2 bomber attacks on Houthi rebels in Yemen during the Gaza war.

Opposition politicians in the UK have been critical of the deal, which would see Britain pay Mauritius £101 million ($136 million) annually for 99 years.

The net cost over the length of the lease would be around £3.4 billion if inflation was factored in, according to the government.

Kemi Badenoch, leader of the opposition Conservative party, said: "Unfortunately on this issue President Trump is right."

"Thank goodness Trump has vetoed the surrender of the Chagos islands," said Nigel Farage, the leader of the hard-right Reform UK party.

But Ed Davey, leader of the centrist Liberal Democrat party, said in a post on X that Prime Minister Keir Starmer should begin to stand up to the US leader.

"This shows Starmer's approach to Trump has failed," he said. "The Chagos seal was sold as proof the government could work with him. Now it's falling apart. It's time for the government to stand up to Trump -- appeasing a bully never works."

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