Donald Trump put his relationship with Sir Keir Starmer under strain last night after accusing the 'far-left' Labour Party of 'blatant foreign interference' in the US election.
Lawyers for the Republican presidential candidate have filed a complaint to the Federal Election Commission, accusing Labour of making 'illegal foreign campaign contributions' which was 'accepted' by Kamala Harris's campaign.
The comments come weeks after Mr Trump and Sir Keir held a two-hour dinner in New York, after which the former president said the Prime Minister was 'very popular'.
Mr Trump's camp cited a now-deleted post on LinkedIn by Sofia Patel, Labour's head of operations, which said 100 current and former staffers were going to be volunteering in battleground states such as North Carolina and Nevada.
Rules about foreigners working in US elections are strict and state that they must volunteer and cannot receive any payment.
It also references a report in the Washington Post that suggested 'strategists linked to Britain's Labour Party have been offering advice to Kamala Harris about how to earn back disaffected voters and run a winning campaign from the centre left'.
The complaint also mentioned reports that Morgan McSweeney, the PM's chief of staff, and Matthew Doyle, Downing Street's director of communications, met with Ms Harris's team at the Democratic National Convention in Chicago over the summer.
Mr Trump harked back to America's Declaration of Independence from British rule in 1776 and said that the UK appeared to have 'forgotten' that the US wanted to be free .
Help from Labour could amount to an 'illegal foreign national contribution', wrote Gary Lawkowski, deputy general counsel for the Trump campaign.
That would be especially the case if 'foreign nationals' are 'exercising direction' over the Labour staffers as could well be the case, the document stated.
Campaign co-manager Susie Wiles said in a statement that Americans will 'once again reject the oppression of big government that we rejected in 1776', another nod to America's rejection of British rule.
She said: 'The Harris campaign's acceptance and use of this illegal foreign assistance is just another feeble attempt in a long line of anti-American election interference'.
In comments that will likely be embarrassing for Sir Keir, Ms Wiles added that the 'far-left Labour Party has inspired Kamala's dangerously liberal policies and rhetoric'.
She wrote: 'In recent weeks, they have recruited and sent party members to campaign for Kamala in critical battleground states, attempting to influence our election'.
Richard Grenell, former US Acting Director of National Intelligence and former ambassador to Germany under Trump, told Newsnight last night: 'We don’t want to have any foreign interference in our elections... so I think this is a pretty open and shut case: don’t interfere in the American elections and you won’t be sued."
Mr Trump has himself faced allegations of illegal foreign interference in his own campaigns.
In 2016, his critics said that he colluded with Russia to win the vote but the inquiry into the affair did not lead to any criminal charges.
A recent book by legendary US journalist Bob Woodward claimed that Mr Trump had met with Vladimir Putin at least 16 times since he left office in 2021.
Elon Musk, the Tesla and X billionaire who has endorsed Mr Trump, said: 'This is illegal.'
There is no evidence that Labour had made any financial contributions to the Democrat campaign.
Downing Street, Mr Doyle and Labour were contacted for comment.
A Labour official told Politico: 'It says a lot about the current level of political discourse on both sides of the Atlantic that an innocuous LinkedIn post from a party staffer has turned into a diplomatic event'.
