A defiant Jeremy Corbyn is expected to face a leadership challenge after vowing to fight on despite the overwhelming vote of no confidence by his own MPs.
Angela Eagle has emerged as the likely challenger to the embattled leader and was the most senior member of the shadow cabinet to resign amid the chaos as scores of members of Mr Corbyn's top team deserted him.
The Labour leader, who will face a difficult session of Prime Minister's Questions in the Commons following the 172 to 40 vote in favour of the no-confidence motion, is expected to receive a warmer reception at a rally of his supporters on Wednesday evening.
Mr Corbyn has insisted he will not quit despite the lack of support from his parliamentary colleagues, instead pointing to his backing among the party's grassroots.
Insisting that the vote by MPs had "no constitutional legitimacy", he said: "I was democratically elected leader of our party for a new kind of politics by 60% of Labour members and supporters, and I will not betray them by resigning."
Crucially, Mr Corbyn also appears to enjoy the support of trade union chiefs.
He will address a rally organised by his backers in the Momentum movement on Wednesday evening, with Public and Commercial Services union chief Mark Serwotka and Fire Brigades Union general secretary Matt Wrack also speaking in support of him.
Unite trade union general secretary Len McCluskey was among the first to rally to Mr Corbyn's support after the no-confidence motion, accusing the MPs of "pointless posturing" and warning they would have to mount a full-blown leadership challenge if they wanted to oust him.
Former shadow business secretary Ms Eagle is tipped to run against him, with Labour MP Mike Gapes saying Mr Corbyn's camp should "fear her" because she is "principled, competent and honest".
But a Labour source loyal to Mr Corbyn insisted "our support is still strong" and any attempt to oust the leader would amount to "shoving two fingers up to democracy".
The source said Ms Eagle "would be the ideal candidate for us" because of her voting record on issues such as backing air strikes against Syria.
"I'm not fearful," the source said.
Labour's leader in Scotland Kezia Dugdale said she would quit if she had suffered the same catastrophic loss of support as Mr Corbyn.
"If I was in his position, if I had lost the confidence of 80% of colleagues I would resign because I could simply not do my job," she told ITV Border.
The mayhem on the Labour benches in the Commons has led to the Scottish National Party claiming it now forms the official opposition - because its Westminster leader Angus Robertson has the support of more MPs than Mr Corbyn.
Senior SNP MP Pete Wishart said there are "obligations to be met to be the official opposition" in the Commons and Labour "now can't meet then".
Referring to Parliamentary rule book Erskine May he said: "For those asking how this is possible it's in Erskine May. The official opposition must be 'prepared to assume office'. Labour can't any more ... Whereas the SNP can."