Donald Trump's campaign has filed a Federal Election Commission (FEC) complaint against the UK's Labour Party, accusing it of "blatant foreign interference" in the US election in aid of the Harris-Walz campaign.
The complaint cites media reports about contact between Labour and the Harris campaign as well as apparent volunteering efforts, arguing that this amounts to illegal "contributions".
The BBC understands that Labour activists campaigning in the US presidential election are doing so in a personal capacity.
The Labour Party has not issued an official response.
Specifically, the complaint cites 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 letter refers to Washington Post reporting that communications were exchanged between the parties and that senior officials have met in private.
Additionally, the complaint cites a social media post on LinkedIn in which a Labour staff member said that "nearly 100" current and former party members will be headed to battleground states in the US.
The post, from Labour Party head of operations Sofia Patel, added that 10 "spots" are available and that "we will sort your housing".
It appears to have since been deleted.
The complaint makes comparisons to 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.
Labour activists' trips were not 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.
It is considered normal for party officials from the UK to be in contact with counterparts in the US.
It also has taken place previously between the UK's Conservative Party and US Republicans.
The BBC has contacted the Harris-Walz campaign for comment.