Donald Trump has filed a legal complaint against Britain’s ruling Labour Party after its activists traveled to the U.S. to support his rival Kamala Harris.

A U.K. Labour staffer revealed on LinkedIn last week that almost 100 party officials were heading across the Atlantic to campaign for the U.S. Democrats in key swing states, ahead of the Nov. 5 election.

Labour insists it is not funding the travel or accommodation for the activists, meaning its efforts remain within strict U.S. federal election rules which stipulate foreign volunteers can’t spend more than $1,000 helping candidates.

But the Trump campaign questioned those sums in its formal letter to the Federal Election Commission, arguing the LinkedIn post suggested Labour could be stumping up costs for the activists.

“Those searching for foreign interference in our elections need to look no further than [the] LinkedIn post,” said the letter, from Trump campaign lawyer Gary Lawkowski. “The interference is occurring in plain sight.”

Lawkowski demanded the FEC launch an immediate investigation into the matter.

He also quipped: “This past week marked the 243 anniversary [sic] of the surrender of British forces at the Battle of Yorktown, a military victory that ensured that the United States would be politically independent of Great Britian [sic]. It appears that the Labour Party and the Harris for President campaign have forgotten the message.”

POLITICO reported last week how U.K. Labour and the U.S. Democrats, including center-left think tanks in Washington and London, have been coordinating in a bid to boost their electoral chances.

The U.K. government has previously insisted the party activists are volunteering in the U.S. on a personal basis, and that their help does not represent the position of the U.K. government. The Labour Party has yet to comment.