Donald Trump's team was handed a private letter warning about Keir Starmer's Chagos Islands deal months before his attack.
It comes as the 47th President lampoons the Prime Minister's plan to hand the Chagos Islands to Mauritius, calling the deal an "act of stupidity".
Taking to Truth Social on Tuesday, Trump said the agreement to hand over the territory was being made “for no reason whatsoever” and cited it as a reason why America should take control of Greenland.
This is the first time Trump has publicly criticised the £3.4bn ($4.6bn) agreement signed in May, which would see the UK hand over sovereignty of the Chagos Islands to Mauritius while keeping the UK-US military base on Diego Garcia on a 99-year lease.
However, a private letter overseen by Facts4EU and Campaign for an Independent Britain (CIBUK) on the eve of Trump's second state visit in September indicates the attack was months in the making.
The private letter warns transferring sovereignty of the Chagos Islands to Mauritius would 'gravely undermine' UK and US security interests
|The private letter handed to an advance party of the President’s entourage (see image below) warned that transferring sovereignty of the Chagos Islands to Mauritius would "gravely undermine" UK and US security interests by threatening the future of the Diego Garcia military base - a facility described as the cornerstone of America's nuclear and naval power projection in the Indian Ocean.
The letter, signed by a cross-party coalition of more than 40 senior MPs, peers, former ministers, and national security leaders, called on the US President Trump to stop the deal.
The group argued that transferring the Chagos Archipelago to Mauritius would place Diego Garcia under the jurisdiction of a state bound by the Pelindaba Treaty, Africa's nuclear-weapon-free zone agreement.
They warned this could create "dangerous legal ambiguity" over US nuclear-capable operations, leaving them vulnerable to attack from China, Russia, or international bodies.
The letter was signed by a cross-party coalition of more than 40 senior MPs, peers, former ministers, and national security leaders
|Notable names that signed the letter - reported on by Facts4EU and shared exclusively with GB News - include former Tory leader Sir Iain Duncan Smith, former home secretary Suella Braverman and Sir Grant Shapps, the ex-defence secretary.
The letter reads: "Mr President, you have built your reputation on refusing to cede strategic ground to those who would weaken America.
"We urge you to use your influence, publicly and privately, and your prerogatives under the US-UK defence agreement of 1966 to oppose this surrender and to ensure that Diego Garcia remains exempt from any constraint under the Pelindaba Treaty."
At the time, then-Foreign Secretary David Lammy batted away these concerns, insisting that the agreement "secures this vital military base for the future".
This position has become harder to defend in light of Trump's latest intervention, where he portrays the deal as a "total weakness" in the eyes of Russia and China.
He also cited the deal as justification to take Greenland, a threat fracturing the European alliance.
Trump wrote on his social media platform: "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."
More From GB News