Lord Cameron held a face-to-face meeting with Donald Trump in Florida as part of a charm offensive designed to secure more US funding for the war in Ukraine.
The foreign secretary visited the former US president at his Mar-a-Lago resort despite previously calling him “protectionist, xenophobic, [and] misogynistic” and denouncing one of his policies as “divisive, stupid and wrong”.
It was the first meeting between a senior minister and Mr Trump since he left office in 2021.
As well as Ukraine, the two men are understood to have talked about the strength of the UK-US relations, the so-called ‘special relationship’, and other foreign policy issues.
Lord Cameron is on a high-profile visit to the US to press Republicans to pass the blocked aid package for Ukraine and will also discuss Israel’s war in Gaza.
It is the latest in a series of interventions made by the foreign secretary over the support for Ukraine’s battle against Russia.
Earlier this year, he warned Congress not to show “the weakness displayed against Hitler” in the 1930s.
That comment drew the ire of right-wing congresswoman and staunch Trump ally Marjorie Taylor Greene, who told him to “kiss my ass” and “worry about his own country”.
Lord Cameron flew to Florida to meet Mr Trump on Monday night.
The Foreign Office insisted that it was “standard practice for ministers to meet with opposition candidates as part of their routine international engagement.”
But it is not the first time Lord Cameron’s decision to meet a presedential candidate has raised eyerbrows.
In 2012, when he was prime minister, he met the Republican candidate for president Mitt Romney during a campaigning and fundraising trip to London.
During his visit, Lord Cameron is hoping for a breakthrough with Republicans who have been blocking a $60bn (over £47.5bn) military aid package for Ukraine, now into the second year of its war with Russia.
He will hold talks with US secretary of state Antony Blinken and key figures across Congress on Tuesday to call them to “change the narrative on Ukraine this year” and release the funding.
He will tell the American leaders that the “success for Ukraine and failure for [Vladimir] Putin are vital for American and European security”.
“This will show that borders matter, that aggression doesn’t pay and that countries like Ukraine are free to choose their own future,” the Foreign Office said.
“The alternative would only encourage Putin in further attempts to re-draw European borders by force, and would be heard clearly in Beijing, Tehran and North Korea.
“US support for Ukraine has massively degraded the military capacity of a common adversary, Russia has lost half of its pre-invasion land combat power, and a quarter of its original Black Sea fleet, while creating jobs at home and strengthening the Western alliance and Nato.”
Republicans within the US Congress have blocked the latest tranche of the package to Ukraine for months. The Republicans in the House of Representatives have demanded concessions on border security before supporting the bill.
The former president, who has claimed he would end the war in Ukraine “in one day”, has long been critical of the huge amount of economic and military aid the US has given Ukraine.
Last month he appeared to soften his tone on Nato in the wake of uproar after he said he would encourage Russia to do “whatever the hell they want” to members who don’t pay their fair share.
In 2015, Mr Cameron described the former Republican president’s “Muslim ban” travel policy as “divisive, stupid and wrong” and criticised his disagreement with Ukraine aid as “not a sensible approach”.
Ms Taylor-Greene’s outburst came in response to an article by Lord Cameron cautioning the US against mirroring “the weakness displayed against Hitler in the 1930s”.
The Foreign Office said the talks with Mr Blinken and other Biden administration officials will also focus on the ceasefire in Gaza and the delivery of more aid to the region.
He will also “push for a full, urgent, and transparent investigation into the terrible events in Gaza” after the “completely unacceptable” deaths of three British men working for aid organisation World Central Kitchen in the Israeli drone strike.
Both Mr Trump and the White House have been approached for comment.
