Donald Trump’s campaign has accused Britain’s governing party of “blatant foreign interference” in the US presidential election over a trip by its activists to help Vice President Kamala Harris’ bid, igniting a spat with one of Washington’s closest allies in the final stretches of the race.

A lawyer for the former president filed a complaint on Tuesday to the Federal Election Commission (FEC) against both the UK’s Labour Party and the Harris campaign, after a Labour staffer wrote a LinkedIn post advertising a trip to the US on which “nearly 100 Labour Party staff” members would campaign for Harris in four key swing states.

But the Trump campaign did not present any new evidence that rules were broken. Labour, and the party’s Prime Minister Keir Starmer, quickly denied that the party funded the trips, insisting the campaigners traveled in a personal capacity at their own expense.

Under FEC rules, foreign nationals are allowed to campaign for a US electoral candidate as long as they remain “an uncompensated volunteer.”

The spat has the potential to sour relations between Trump and Starmer, a center-left politician who has remained neutral on the upcoming election and repeatedly said his government would work with whichever candidate wins.

Starmer told reporters on Wednesday that any Labour Party staffers involved in the trip were there in a personal capacity, adding: “They’re doing it in their spare time, they’re doing it as volunteers, they’re staying, I think, with other volunteers over there.”

“That’s what they’ve done in previous elections, that’s what they’re doing in this election and that’s really straightforward,” Starmer said.

But the Trump campaign elevated the dispute in heightened language on Wednesday. Its co-manager, Susie Wiles, said in a statement that “Americans will once again reject the oppression of big government that we rejected in 1776” and described the center-left Labour as a “far-left” party that has “inspired Kamala’s dangerously liberal policies.”

Trump seeks to deflect election interference claims

The Trump campaign’s complaint stems from a since-deleted LinkedIn post in which Sofia Patel, a Labour head of operations, wrote: “I have nearly 100 Labour Party staff (current and former) going to the US in the next few weeks heading to North Carolina, Nevada, Pennsylvania and Virginia,” four states that could decide the upcoming election.

“I have 10 spots available for anyone available to head to the battleground state of North Carolina – we will sort your housing,” Patel’s post added.

FEC rules state that an “individual who is a foreign national may participate in campaign activities as an uncompensated volunteer. In doing so, the volunteer must be careful not to participate in the decision-making process of the campaign.”

The Trump campaign’s complaint does not contain any evidence that the individuals were compensated; instead it references the LinkedIn post and various media reporting, asking the FEC to investigate further.

And Labour has insisted that it did not fund any aspects of the trip. A party spokesperson told CNN: “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.”

The party also pushed back at the Trump complaint’s suggestion that two high-ranking Labour staffers, who attended the Democratic National Convention in August, were there to provide assistance to the Harris campaign.

CNN has requested comment from the Harris-Walz campaign on Trump’s letter. The FEC declined to comment.

It is commonplace for Labour to send a delegation to the DNC, and members and staff for the Labour Party – a large political party with both a national office and hundreds of local offices – have historically traveled to the US to campaign in elections without attracting media attention.

Trump has frequently attempted to deflect allegations that he has benefited from foreign electoral interference from countries including Russia. The US intelligence community said in a landmark report in 2021 that the Russian government meddled in the 2020 election with an influence campaign “denigrating” Joe Biden and “supporting” Trump.

Starmer’s center-left Labour Party swept to power in Britain in a July general election, and has long held an informal but friendly relationship with the Democratic Party.

But Starmer has repeatedly insisted his government will work with whoever wins November’s presidential contest, and met with Trump in New York during the United Nations General Assembly last month.

“I spent time in New York with President Trump, had dinner with him, and my purpose in doing that was to make sure that between the two of us we established a good relationship, which we did, and I was very grateful to him for making the time,” Starmer said on Wednesday.

This story has been updated with additional developments.