Mr Trump’s campaign has claimed Labour made ‘illegal foreign national contributions’ to his election rival, Kamala Harris, as part of an official complaint to US authorities

Sir Keir has insisted any members of his party teaming up with the Democrats and Ms Harris’s campaign were in the US on an entirely voluntary basis in their spare time

During business questions in the Commons today, Mr Philp called for a debate on foreign interference in elections.

He suggested Labour was ‘damaging’ the UK’s national interest, telling MPs: ‘I understand that over 100 Labour Party staffers are enjoying themselves in the US currently, in the presidential election under way now.

‘Ministers have claimed this is all spontaneous, all organised themselves, all paid for themselves as well.

‘But that claim appears – if I can put it politely – grossly implausible now it’s emerged in a now-deleted social media tweet or post, that the whole thing was arranged by the Labour Party’s director of operations.’

Speaking on his Political Currency podcast with former chancellor George Osborne, Mr Balls said: ‘It’s naive and gauche.

‘These kinds of visits have been happening for decades but this is a different election. You would think people would have known from 2020 how delicate this is.’

He added: ‘When you start organising a sort of a posse of campaigners as a foreign party, now a foreign government, an alarm bell should have rung, and there will be people kicking themselves, thinking ‘I saw that and I didn’t do anything’.

‘If you’re in opposition for years, you get used to the idea that nobody cares that much what you do, and when you’re in government, suddenly it changes overnight.

‘They won’t make this mistake again, but it’s definitely a mistake, and somebody should have spotted it earlier.’

Earlier, pictures emerged of a Labour MP posing with Kamala Harris campaign literature in the swing state of New Hampshire as she boasted of doing ‘my bit to stop Trump’.

Ruth Cadbury, MP for Brentford and Isleworth, was among the cohort of MPs, aides and other Labour members who have headed to the US to help the Democrats, their unofficial sister party.

The Transport Committee chairwoman travelled to America at the end of September, after the Labour Party conference but before the Commons returned from its summer recess.

Others had already been to the US as part of a delegation to the Democrat National Convention in August.

Ruth Cadbury, MP for Brentford and Isleworth, was among the cohort of Labour figures who have headed to the US to help the Democrats

Among the others who travelled was Mike Tapp, the new MP for Dover and Deal in Kent

Mr Trump, the Republican candidate, this week accused Labour of ‘blatant foreign interference’ in the upcoming vote

A complaint made by Trump’s campaign to the US Federal Election Commission also cited a visit by the PM’s now chief of staff Morgan McSweeney

There is no suggestion that Ms Cadbury has broken any rules or laws.

A spokesman for the MP told MailOnline: ‘During the parliamentary recess Ruth took a holiday to Boston and New Hampshire.

‘While visiting Ruth combined the trip with a day’s door-knocking in New Hampshire. Ruth fully paid for and organised the trip herself.’

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 has 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.

He was part of the senior Labour entourage who went to the DNC in Chicago, which his £3,000 bill covered by the Progressive Policy Institute (PPI) think tank.

It also paid for the Labour director of communications Matthew Doyle to attend.

Mr Tapp told the Telegraph after his trip that he had met several members of Kamala Harris’s  team during what he called a ‘political holiday’ on X. It was his second trip of the year, having previously visited the campaign in January.

He told the newspaper: ‘We went back out after our landslide victory to impart some of our knowledge as to what we learnt on the campaign trail, and to look at what they’re doing with their campaign.

Mr Tapp told the Telegraph after his trip that he had met several members of Ms Harris’s team during what he called a ‘political holiday’ on X

‘One of the big messages that I was taking over there was to ensure that with their campaign, they do what we did, and that is, listen to what working class, hard-working people want.

‘Don’t ignore them. And for me in Dover and Deal, a perfect example of that is worries and concerns around immigration.

‘These people have valid concerns. They’re not racist, we must drop that sort of rhetoric and deal with these issues, just like Keir Starmer and Yvette Cooper are doing.

‘And it does look like Kamala and her team are going down that route as well, which is great.’

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.

Mr Farage also faced questions over his own efforts to help Mr Trump. The Clacton MP raised eyebrows by travelling to attend the Republican National Convention in Milwaukee in July

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.’

However, Mr Farage also faced questions over his own efforts to help Mr Trump. The Clacton MP raised eyebrows by travelling to attend the Republican National Convention in Milwaukee in July straight after the State Opening of Parliament in London.

The Milwaukee event was staged nearly 4,000 miles from Mr Farage’s seaside constituency in Essex. But he said he was attending in order to ‘show support’ for Mr Trump following an alleged assassination attempt.

Quizzed about whether he was neglecting his commitment to Clacton by heading across the Atlantic, he replied: ‘I’m allowed to come to America on a trip like this, particularly in these circumstances.

‘I had planned to come originally to America and be here for a few months. But I made a decision, ‘no, I’m going to run for Clacton’.

‘Given the circumstances, it was right that I came.’

The almost £33,000 tab for the trip for Mr Farage and an aide, was paid for by a private donor, Christopher Harborne, a tech investor based in Thailand, according to the register of MPs financial interests. There is no suggestion of wrongdoing.