Jeremy Corbyn has refused to quit as Labour leader despite losing an no confidence vote overwhelmingly by 172-40.
The embattled Labour leader said he would not "betray" Labour members who elected him last year.
Mr Corbyn has faced a near universal revolt among MPs determined to dislodge him and more than 50 MPs quit front bench roles over the past three days. The no confidence vote triggered another wave of resignations.
But despite the massive vote against him by Labour MPs tonight, Mr Corbyn is to try and fight on - insisting his mandate from party members' is more important than MPs' support.
Senior Labour sources indicated Tom Watson and senior MP Angela Eagle are discussing how to bring about the leadership challenge all sides accept is now inevitable.
But in a statement issued within minutes of the result, Mr Corbyn rejected the result out of hand.
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. Today's vote by MPs has no constitutional legitimacy.
"We are a democratic party, with a clear constitution. Our people need Labour party members, trade unionists and MPs to unite behind my leadership at a critical time for our country."
There were 13 MPs who did not take part in today's vote - reportedly including Rachael Maskell, a member of the shadow cabinet as of yesterday morning who has declined to express confidence in the leader.
Mr Corbyn gathered his rag-tag new shadow cabinet in an attempt to portray unity - but he still has at least seven top tier vacancies and around 30 more junior positions to full.
A Labour spokesman said: "Following the ballot conducted today, the Parliamentary Labour Party has accepted the following motion: That this PLP has no confidence in Jeremy Corbyn as Leader of the Parliamentary Labour Party."
In response, Mr Corbyn pleaded with Labour MPs to help him fight the Tories.
He said: "The government is in disarray. Ministers have made it clear they have no exit plan, but are determined to make working people pay with a new round of cuts and tax rises.
"Labour has the responsibility to give a lead where the government will not. We need to bring people together, hold the government to account, oppose austerity and set out a path to exit that will protect jobs and incomes.
"To do that we need to stand together. Since I was elected leader of our party nine months ago, we have repeatedly defeated the government over its attacks on living standards."
More resignations emerged almost as soon as the no confidence vote has been lost - starting with shadow home office minister Lyn Brown, swiftly followed by shadow business minster Kevin Brennan.
Christina Rees left the shadow justice team, swiftly followed by Sarah Champion who quit the home office team. Clive Efford then left the shadow culture, media and sport team.
Close ally Andy Slaughter quit the front bench this morning and tonight said Mr Corbyn was not capable of doing the "difficult job" of leading a major party - particularly at a time of "national crisis".
Mr Slaugher told BBC Radio 4 tonight: "I don't think he has the experience or the skills to the leader of a major political party."
Scottish Labour leader Kezia Dugdale tonight told BBC Scotland: "If I lost the confidence of 80 per cent of my parliamentary colleagues I could not do my job."
But PCS leader Mark Serwotka, asked if Mr Corbyn should step aside, said: "Absolutely not."
Unite leader Len McCluskey tonight told the Guardian: "If anyone wants to change the Labour leadership, they must do it openly and democratically."
If Mr Corbyn refuses to go, a challenge can be forced by 51 MPs officially writing to the National Executive Committee (NEC) of the Labour.
This would trigger a new leadership election and it is hotly disputed as to whether Mr Corbyn would get on the ballot by right as the incumbent.
Party rules say candidates would need 37 nominations to get on the ballot - and today's vote suggest he has such support making a debate over the rules academic.
Earlier, footage of the new shadow cabinet shows a very awkward looking Mr Corbyn sat next to his deputy Mr Watson - who has refused to condemn the rebellion - at the shadow cabinet table.
At one point the leader beckons over aide Seumas Milne and whispers: "I'm not sure this is a good idea."
In other developments today, Pat Glass, who was promoted to education secretary after Lucy Powell quit yesterday, announced that she is standing down from parliament at the general election. She said the EU referendum campaign had taken a toll on her and her family.
Mr Corbyn has refused to countenance resigning under any circumstances despite the draining away of what little support he had from mainstream Labour in Parliament.
Last night he addressed a crowd of thousands of people in Parliament Square, some of whom had banners and t-shirts branded 'eradicate the right wing Blairite vermin', vowing to fight on.
The developments have prompted new questions about whether Labour is broken and must split - and one former MP today admitted "separation and divorce are now inevitable".
Dame Margaret Hodge, who tabled today's no confidence motion, pleaded with the Labour leader to recognise he was destroying the Labour Party and resign before it came to a split.
She told the BBC: "I think that the reaction last night at the parliamentary Labour party was unprecedented.
"I couldn't believe the strength of feeling, the overwhelming rejection of Jeremy as our leader, and the pleading with him that he should actually consider his position and go with dignity.
"No one individual is greater than the party. Did we use the referendum as an excuse? Well I think the referendum was actually a test of leadership and he failed it."
She added: "I looked at the pictures of the demonstrations last night, those weren't Labour party members.
"They were members of the Socialist Workers Party, which is an extreme left grouping, and they were members of Momentum, which is also an extreme left grouping."