Donald Trump's campaign has accused British Prime Minister Keir Starmer's Labour Party of "blatant foreign interference" in the U.S. presidential election after its volunteers travelled to the United States to help campaign for Kamala Harris.

The campaign has filed a complaint with the Federal Election Commission in Washington, calling for an immediate investigation into what it called "apparent illegal foreign national contributions made by the Labour Party of the United Kingdom and accepted by Harris for President".

The complaint cited media reports and a now deleted LinkedIn post from Sofia Patel, head of operations at Britain's Labour Party, who wrote that "nearly 100" current and former Labour party staff would be travelling to the U.S. in the coming weeks to help elect Harris, the Democratic vice-president.

"Those searching for foreign interference in our elections need to look no further than [the] LinkedIn post," the letter of complaint said. "The interference is occurring in plain sight."

The centre-left Labour Party, which sees the U.S. Democrats as its sister party, swept to power in July. Starmer has since sought to build ties with Trump, meeting him at his Trump Tower during a visit to New York in September.

Starmer, travelling on a flight to Samoa, told reporters he did not expect the complaint to strain relations with Trump if he wins the election on Nov. 5, adding Labour volunteers had gone to pretty much every U.S. 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," he said.

"That's what they've done in previous elections, that's what they're doing in this election and that's really straightforward."