A row over alleged UK interference in the US election deepened last night, despite Sir Keir Starmer's attempts to defuse the transatlantic fall-out.

Donald Trump this week accused Labour of 'blatant foreign interference' in the upcoming vote on November 5, following a call for 100 current and former party staffers to volunteer for his rival Kamala Harris in battleground states.

It is not illegal for foreign nationals to serve as campaign volunteers in US elections, but only if they are not compensated in any way. However, Mr Trump accused the Labour Party of making 'illegal' foreign contributions.

A complaint made by his campaign to the US Federal Election Commission also cited a visit by the PM's now chief of staff Morgan McSweeney and his director of communications Matthew Doyle to the Democratic National Convention (DNC).

Mr McSweeney, who was then the Prime Minister's political strategist, attended the event in Chicago in August, with, it emerged yesterday, the central party picking up the tab.

Labour denied that he advised the campaign team of Ms Harris, the Democratic nominee, which would potentially have been illegal under US electoral law.

But that was contradicted by remarks from Dover MP Mike Tapp, who was part of the delegation to the DNC – the US equivalent of a UK party conference.

After the visit, he said: 'We went back out after our landslide victory, to impart some of our knowledge as to what we learnt on the campaign trail, and look at what they are doing with their campaign.'

In their complaint, Mr Trump's legal team cited a now deleted LinkedIn post by Sofia Patel, Labour's head of operations, which said: 'I have ten spots available for anyone available to head to the battleground state of North Carolina – we will sort your housing.' This raised questions because sorting housing could be viewed as compensating campaign volunteers if Labour covered the costs.

John Lamont, the shadow Scottish secretary, said: 'It's a diplomatic car crash by this Labour Government.

'If Donald Trump were to win the election in a few weeks, how on earth is the Prime Minister going to rebuild that relationship with one of the most important countries in the world?'

Meanwhile, Reform UK leader Nigel Farage said: 'This is a – politically – very, very, stupid thing to have done.'

Mr Trump's allies yesterday piled pressure on Downing Street.

Richard Grenell, a former Director of National Intelligence who is tipped to be in Mr Trump's cabinet if he wins, told the BBC: 'I think whenever a foreign government tries to interfere in an election, that's desperation.

'I think this is a pretty open and shut case – don't interfere in the American elections and you won't be sued.' Sir Keir and Cabinet ministers scrambled to justify Labour's involvement by insisting that campaigners did not receive compensation.

Speaking to reporters on his way to a Commonwealth summit in Samoa, the PM said: 'Volunteers have gone over pretty much every election.

'They're doing it in their spare time, they're doing it as volunteers, they're staying, I think, with other volunteers over there.'

And asked about it at Prime Minister's Questions yesterday, Deputy Prime Minister Angela Rayner said: 'People in their own time often go and campaign, and that's what we've seen.

'It happens in all political parties. People go and campaign and do what they want in their own time and with their own money.'

Yesterday it emerged the Labour MP for Brentford and Isleworth Ruth Cadbury was among those campaigning for Ms Harris. She posted pictures with campaign literature in New Hampshire in September, saying that she was going door-knocking to do 'my bit to stop Trump winning'. Ms Cadbury said she paid for the trip herself.

Party sources last night said many people associated with Labour were now scrapping plans to travel to the US to campaign as volunteers due to the row.

A Labour Party spokesman said: 'It is common practice for campaigners of all political persuasions from around the world to volunteer in US elections. Where Labour activists take part, they do so at their own expense, in accordance with the laws and rules.'