Train drivers' union ASLEF could cut its historic ties with the Labour Party amid furious rows between Sir Keir Starmer and the Left.
The Mirror understands ASLEF's conference in May will debate a motion for the union to disaffiliate from Labour in a move which would dramatically end a relationship stretching back almost 120 years.
The news comes as a bitter dispute plays out between the Labour Leader and Unite General Secretary Sharon Graham over the party's refusal to back bin workers in Coventry on strike over pay.
Sir Keir warned the union boss he would not be "influenced by threats" after Ms Graham underlined that the union could slash funding.
A trade union source said ASLEF's motion to disaffiliate was tabled by left-wingers who fear Sir Keir is sidelining the organisation in the wake of Labour's disastrous defeat in 2019 under Jeremy Corbyn.
The former leader, who remains suspended as a Labour MP over his response to the equalities watchdog's report into anti-Semitism in the party, had a close relationship with ASLEF, in particular Tosh McDonald who quit the party last March.
"There has been a sense since Keir Starmer became leader that unions have been pushed to the periphery and that has left some people disenchanted," said the source.
ASLEF would be the second union to officially sever links after bakers' union BFAWU voted to disaffiliate in September, claiming a "factional internal war led by the [Labour] leadership".
Among the smaller unions, ASLEF has been affiliated to the Labour Party since 1903 but its General Secretary, Mick Whelan, is a high-profile figure in the current labour movement as chairman of (the Trade Union and Labour Party Liaison Organisation) - a key group which links the party with union officials on issues of policy, campaigning and rule changes.
Sir Keir became embroiled in an angry battle with general secretaries at TULO meetings at Labour's annual conference in Brighton last year over a controversial shake-up of leadership election rules.
Mr Whelan is said to be personally supportive of affiliation but around 80 union delegates will decide on the motion in Bournemouth.
The move to break ties "could go either way", it is understood, and comes 12 months after the union reaffirmed its relationship with Labour.
The union source said the motion was being driven by "some on the Left of the Labour Party and some to the Left of the Labour Party", and that delegates from Scotland were keen to build a relationship with Nicola Sturgeon's SNP.
"What will happen I don't know," the source added.
A Labour source supportive of Sir Keir urged the union to stay in the tent and said cutting ties would weaken the union.
They said: "I think everyone in the Labour Party will hope that the people in ASLEF who want to stay affiliated to Labour win the vote.
"That's in Labour's interests and will ensure ASLEF continues to have a strong voice.
"Of course, unions have other members who are anti-Labour and want to break the link.
"We need to stop them achieving the Tory dream of separating the industrial and political wings of the labour movement."
An ASLEF spokesman confirmed the motion to disaffiliate would be debated, saying: "This is a matter for our annual conference and our delegates in Bournemouth in May will make the decision."
