Unite boss Len McCluskey has warned Sir Keir Starmer not to move Labour too far from the left as the union moved to cut affiliation money.
The union, which is Labour’s biggest financial donor, is reducing affiliation by 50,000, or 10%, after a vote of its executive on Tuesday.
Mr McCluskey, an ally of Sir Keir’s predecessor Jeremy Corbyn, warned that Unite’s multimillion-pound funding could be cut if the new leader undertakes a too drastic change of course.
Ahead of the vote, Mr McCluskey told BBC Newsnight: “I have no doubt if things start to move in different directions and ordinary working people start saying ‘Well, I’m not sure what Labour stands for’…
“But I don’t see at the moment any dramatic move to disaffiliate from the Labour Party. The Labour Party is our party.”
However, he was particularly critical of Labour’s payout to whistleblowers over the party’s handling of anti-Semitism under Mr Corbyn.
“I think funding arrangements is undoubtedly an issue that may come up,” Mr McCluskey added.
Sir Keir’s spokesman said: “Unite have taken their decision. We have seen Len’s remarks and we acknowledge that.
“But Labour will continue to take the decisions that are in the best interests of the country.
“Keir’s focus and the entire Labour movement’s focus is restoring trust in the Labour Party and winning in 2024.”
Asked if it could benefit Sir Keir to appear to have distance from Unite, his spokesman said: “Len has taken his decision, Unite has taken its decision; it is for others to speculate and analyse that decision.”
He added: “The whole Labour Party needs to take responsibility for the fact that we have lost four elections in a row.”
Unite insists its move is aimed at “nurturing emerging talent” as funding is switched to other parts of the Labour movement.
The union said it would still remain Labour’s biggest affiliate.