The Trump campaign complaint cites this, as well as newspaper reporting that Labour-linked individuals have travelled to the US to campaign for Harris.

That reporting, the complaint alleges, creates a “reasonable inference that the Labour Party has made, and the Harris campaign has accepted, illegal foreign national contributions”.

The complaint also refers to Washington Post reporting that communications were exchanged between the parties and that senior officials have met in private.

The Trump campaign has also made comparisons with an international programme in 2016 in which the Australian Labor Party, or ALP, sent delegates to help with Bernie Sanders’ campaign.

In that instance, however, the ALP paid for flights and daily stipends. The party and the campaign were each handed down civil penalties of $14,500.

Activists’ trips by the UK Labour Party have not been organised or funded by the party, it is understood from party officials.

Foreign nationals are permitted to serve as campaign volunteers as long as they are not compensated, according to FEC rules.

A former Labour activist who volunteered for Barack Obama’s presidential campaign in 2008 called Trump’s allegations “nothing more than a political stunt”.

Matthew McGregor told BBC Radio 4’s Today programme that the most a foreign volunteer could receive was a bottle of water or some pretzels.

In reference to Patel’s offer to “sort” the housing of those who went on the trip, Mr McGregor said it was common for Democratic activists to offer their spare rooms or sofas to volunteers.

Speaking to the Today programme himself, one Trump ally said the suggestion of interference was more of a “perception problem than a legal one”.

Chris Ruddy, CEO of Newsmax Media, added that story was more important in the UK than in the US currently, but as the election got closer it “could become a bigger issue”.