Jeremy Corbyn is facing intense pressure to give Labour MPs a free vote on air strikes in Syria, with his shadow cabinet openly split on whether to back David Cameron's call for military action.
Deputy leader Tom Watson became the latest leading figure to speak out in support of bombing Islamic State (IS) in its Syrian heartland – putting him directly at odds with the party leader.
Senior figures on both sides played down the prospect of mass resignations if the shadow cabinet does not agree to give MPs a vote when it meets on Monday – something Mr Corbyn has previously rejected.
But with more than half his top team now against him on the issue, the Labour leader may have little choice but to agree if he wants to stave off a full-scale mutiny.
While allies of Mr Corbyn appealed for calm within the Labour ranks, critics on the backbenches openly voiced their contempt for his "weak" leadership and urged him to step down.
Even some sympathetic MPs admitted that he would be unable to carry on if it became clear that he was a "liability" to the party's chances at the next general election.
The incendiary row was triggered by Mr Corbyn's decision on Thursday to release a letter declaring he could not support RAF air strikes in Syria - seen by some MPs as an attempt to pre-empt Monday's meeting.
However Mr Watson backed shadow foreign secretary Hilary Benn - who was reportedly briefed by intelligence chiefs on the threat from IS – who said there was a "compelling" case for extending RAF airstrikes, currently restricted to Iraq, into Syria.
"I think there is an imminent terrorist threat being directed from Syria," he told the BBC. "Hilary Benn gave a very clear explanation that he thinks there is an imminent security threat to the UK and I agree with him on this."
With allies of Mr Corbyn warning critics would face the wrath of grass roots activists unless they fell into line, Mr Watson made clear he had no intention of resigning – pointing out that he also had been directly elected by party members.
"I am the deputy leader of the Labour Party with a mandate," he said.
Mr Benn also said that he intended to carry on in the shadow cabinet and suggested that a free vote may be the only way out of the impasse.
"The shadow cabinet will continue its discussions on Monday and it may be that that is where we end up," he said.
Shadow chancellor John McDonnell, one of Mr Corbyn's closest allies, appealed for calm, insisting the party was "working through the issues".
"Don't mistake democracy for division," he wrote on Twitter.
However there was no disguising the anger of critics on the Labour back benches, with former minister John Spellar denouncing Mr Corbyn's behaviour as "unacceptable".
"How does Jeremy Corbyn and his small group of tiny Trots in the bunker think they've got the unique view on it all?" he said. "If anyone should resign after this incident, it should be Jeremy Corbyn."
Another ex-minister, Fiona Mactaggart, said Mr Corbyn's leadership was "weak" and "unsustainable".
Asked on BBC Radio Berkshire if she thought he should quit, she said: "I think that would be a sensible strategy because I think that the division at the moment is causing real problems."
Even fellow leftwinger Paul Flynn admitted the party was in a "terrible mess" and that Mr Corbyn may be unable to carry on as leader.
"I've said to Jeremy if you end up like Michael Foot and Gordon Brown and Miliband as a liability to the party, if you are far less popular than when we are coming up to an election you have got to go. I believe that Jeremy understands that," he told BBC Radio 4's The World at One.