LONDON — Just as Keir Starmer was scrambling to smooth things over with Donald Trump, the U.S. president fired an unexpected rocket the British prime minister’s way.

London awoke to a Truth Social post from Trump slamming Britain’s decision to hand control of the Chagos Islands — home of a joint U.K.-U.S. military base at Diego Garcia — to Mauritius.

The British government had long thought the deal was squared with the U.S. administration, but Trump decried it as an act of “great stupidity” that will only embolden Russia and China.

The intervention is a fresh nightmare for Starmer’s government, which was already digging deep to maintain the links the prime minister has painstakingly built with the White House in a week Trump vowed to slap tariffs on the U.K. and European allies who oppose his plan to forcibly acquire Greenland.

Officials received no advance warning of Trump’s intervention — which played right into the hands of domestic opposition parties who have been campaigning against the deal for months.

Starmer’s government was outwardly bullish on Tuesday, with his spokesman insisting that “the U.S. supports the deal.” A bill enacting the transfer is currently making its way through the parliamentary process.

However, ministers confirmed they would make fresh efforts to shore up U.S. support for the Chagos agreement in the coming days. Starmer will now have to strain every sinew to get back on an even keel with his unpredictable counterpart. 

What gives?

Trump’s apparent change of heart follows assiduous lobbying over the deal’s potential risks on both sides of the Atlantic.

In the U.K., the campaign against the Chagos agreement was led by politicians from the right, citing concerns over Chinese influence in the region. They are now claiming victory.

U.S. officials have received representations from Nigel Farage, the populist leader of Reform UK, and Tory figures, including Ross Kempsell, a peer and former aide to Boris Johnson. GB News reported Tuesday that a letter from skeptical British lawmakers was handed to Trump’s team during his state visit to the U.K. in September.

One U.K. defense analyst with U.S. links, granted anonymity to speak candidly, said: “Every single China hawk in D.C. was against the deal.”

Sophia Gaston, a research fellow at King’s College London, said U.S. institutions, which had been working on the negotiations with the U.K. were “supportive” of the deal and that Trump “was happy to wave it through in May as a gesture of trust and goodwill towards the special relationship.”

But she added: “There was always an element of fragility to the president’s support, however, because it’s a deal that’s all based around a respect for international law, and he prioritizes hard power in the national interest.”

A British official, not authorized to speak on the record, did not dispute this. “Pentagon and the State Department looked hard at this and concluded the deal was the best available outcome to secure vital U.S. interests,” they said.

Fresh lobbying push

Foreign Commonwealth and Development Office Minister Stephen Doughty acknowledged Tuesday that the U.K. would now have to lobby the U.S. afresh.

“We will, of course, have discussions with the administration in the coming days to remind them of the strength of this deal and how it secures the base,” he told MPs.

Starmer’s spokesman told reporters the parliamentary process to enact the Chagos treaty would continue as planned, while Mauritius’ Attorney General Gavin Glover issued a statement stressing that it still expects the transfer to go ahead. Campaigners had long argued that Britain’s custody of the archipelago — including the forcible expulsion of Chagossians to make way for the base in the 1960s — was a hangover from its days as a global empire.

Glover said: “The sovereignty of the Republic of Mauritius over the Chagos Archipelago is already unambiguously recognised by international law and should no longer be subject to debate.”

Gaston argued that it would still be “possible” for Starmer to persuade Trump to resume his backing, but warned that the price of doing so could be helping to find a solution to his standoff with Europe over Greenland — or allowing the president to “save some face” on his heavily-criticized Board of Peace for Gaza.

The row poses wider questions for Starmer too. The British prime minister, a human rights lawyer by profession, has described international law as his “lodestar,” and took considerable domestic flack for sticking to his guns on the Chagos deal.

Callum Miller, foreign affairs spokesman for the Liberal Democrats, urged a tougher approach, telling the House of Commons, “we must show President Trump that his actions have consequences” and “we should take no options off the table” when dealing with him. 

But protestations from opposition MPs are unlikely to dissuade Starmer from his settled course of striving for common ground with Trump and raising differences in private.

As one senior Labour MP put it: “It’s presidential trolling. Best not to rise to it.”