The Unite union has threatened to withdraw its funding from the Labour party over a council pay dispute.
Sharon Graham, the union’s general secretary, issued the warning as bin lorry drivers in Labour-run Coventry continued a series of walk-outs.
But a spokesperson for Labour said the party was now “under new management” and would not give in to “threats”.
The Coventry dispute has been ongoing since January and has led to the council hiring private drivers and setting up collection sites so local residents can drop off their rubbish.
In an escalation of the row, Graham tweeted on Wednesday night: “Let me be very clear - the remaining financial support of Labour Party is now under review. Your behaviour and mistreatment of our members will not be accepted. It’s time to act like labour, be the party for workers.”
Unite currently pays £1m to Labour as an affiliation fee, plus hundreds of thousands pounds more in political donations, making it the party’s largest funder.
In response to Graham’s comments, a Labour spokesperson said: “We’re not going to get into the specifics of this dispute. Keir Starmer’s Labour Party will always act in the public interest.
“These sort of threats won’t work in Keir Starmer’s Labour Party. We would have hoped that Unite would have got the message that the Labour Party is under new management.”
The extraordinary public spat is further evidence of worsening relations between Unite and Labour.
In an interview with The Guardian in December, Graham - who replaced Len McCluskey as Unite boss last year - hinted that the union could cut the amount of money it gives Labour.
She said: “There’s a lot of other money that we use from our political fund where, actually, I’m not sure we’re getting the best value for it”
Under McCluskey, who was a staunch supporter of Starmer’s predecessor Jeremy Corbyn, Unite ploughed millions of pounds into the party’s coffers.