Donald Trump has launched an extraordinary attack on Sir Keir Starmer, warning the Prime Minister he is making “a big mistake” and demanding the Chagos deal is torn up. The US President took to the social media platform Truth Social to blast the controversial agreement, telling Sir Keir: “DO NOT GIVE AWAY DIEGO GARCIA!” Mr Trump warned: “Prime Minister Starmer is losing control of this important Island by claims of entities never known of before. In our opinion, they are fictitious in nature.” The intervention piles pressure on Sir Keir over his decision to hand sovereignty of the Chagos Islands to Mauritius under a 99-year lease for the Diego Garcia military base.

Mr Trump also said in his message: “I have been telling Prime Minister Keir Starmer, of the United Kingdom, that Leases are no good when it comes to Countries, and that he is making a big mistake by entering a 100 Year Lease with whoever it is that is ‘claiming’ Right, Title, and Interest to Diego Garcia.” The President said Diego Garcia, strategically located in the Indian Ocean, could be crucial if America needs to take military action against Iran. He warned: “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.”

Mr Trump added the threat could be “an attack that would potentially be made on the United Kingdom, as well as other friendly Countries.” Despite the stark warning about Britain’s security, the President insisted: “Our relationship with the United Kingdom is a strong and powerful one, and it has been for many years.”

But he issued a dire prediction about the Chagos deal, saying: “This land should not be taken away from the U.K. and, if it is allowed to be, it will be a blight on our Great Ally.” Mr Trump concluded saying that the US “will always be ready, willing, and able to fight for the U.K., but they have to remain strong in the face of Wokeism, and other problems put before them.”

The extraordinary intervention comes as the Chagos deal faces mounting opposition at home and abroad. The Conservatives have branded it a “catastrophic surrender” that hands a strategic military asset close to a Chinese ally while costing British taxpayers billions.

Sir Keir was urged to see sense and scrap the “appalling” deal altogether, which saw the country signing a £3.4bn agreement in May. The deal would see the UK retaining control of the diego Garcia military base, alongside the US, while leaving the islands to Mauritius.

Speaking online, the Tories said Trump “was right” with Leader Kemi Badenoch saying the deal was “an act of great stupidity” and the vast payment to Mauritius “complete self-sabotage.”

She added: “Chagos is a strategic asset. If our closest ally is saying this, the PM should listen. It’s time to kill this terrible deal.”

Conservative Shadow Foreign Secretary Dame Priti Patel said: “President Trump has once again publicly rebuked Keir Starmer and his Government over their ill-judged, unnecessary and expensive Chagos Surrender. This is an utter humiliation for Starmer.

“It’s time Starmer finally saw sense, u-turned and scrapped this appalling deal altogether. Giving up British sovereign territory to an ally of China and paying for the privilege is irresponsible and reckless and is clearly undermining our relationship with our most important ally.

“While Starmer, Labour and their leftie lawyer friends threaten to undermine our security and defence, the Conservatives will keep fighting against the Chagos Surrender for as long as it takes.”

Mr Trump’s warning that the claimants to Diego Garcia are “fictitious in nature” echoes concerns raised by critics that Mauritius has no legitimate historical claim to the islands.

The Chagos Islands were separated from Mauritius before it gained independence, and some Chagossians oppose the islands being handed to Mauritius rather than being allowed to return under British sovereignty.

Mr Trump’s public rebuke of Sir Keir marks a significant escalation in international pressure over the deal and raises questions about whether it can proceed without full American support.

It also comes just hours after the US Department of State said it “supports the decision of the United Kingdom to proceed with its agreement with Mauritius concerning the Chagos archipelago”.

In January, Mr Trump had already attacked the Chagos deal in a different Truth Social post, saying the move as an “act of great stupidity” – months after he and senior US officials endorsed it.

A FCDO spokesperson said: “The deal to secure the joint UK-US military base on Diego Garcia military is crucial to the security of the UK and our key allies, and to keeping the British people safe. The agreement we have reached is the only way to guarantee the long-term future of this vital military base.”