A defiant Jeremy Corbyn has said he will not "betray" his grassroots supporters by resigning following an overwhelming vote of no confidence by Labour MPs.

The party leader said the vote by the Parliamentary Labour Party on a motion tabled by the veteran backbencher Dame Margaret Hodge had no "constitutional legitimacy" under party rules.

Although there was no official announcement of the voting figures, sources said that it was 172 to 40 in support of the motion with four spoilt papers.

Unite trade union general secretary Len McCluskey was among the first to rally to Mr Corbyn's support, accusing the MPs of "pointless posturing" and warning they would have to mount a full-blown leadership challenge if they wanted to oust him.

But party grandees David Blunkett and Jack Straw tore into him, warning Labour would be "annihilated" if he carried on while the party's leader in Scotland, Kezia Dugdale, said she could not do her job if she had lost the confidence of 80% of her MSPs.

The vote came as Mr Corbyn continued to struggle to put together a new shadow cabinet following the mass walk-out of more than half his top team, with more shadow ministers announcing their resignations.

In a statement, Mr Corbyn 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."

Mr Corbyn's determination to fight on means the rebels will have to mount a formal leadership challenge if they are to stand any chance of getting rid of him, knowing that defeat would simply leave him more strongly entrenched in his position..

Speculation has been rife that deputy leader Tom Watson or former shadow business secretary Angela Eagle - who quit the shadow cabinet on Monday - could run as a "unity" candidate.

Mr Corbyn's supporters are confident that he will win out in a ballot of grassroots activists who swept him to the leadership last year and who will decide the outcome of any contest.

His team insist that if there is a challenge, he will automatically be on the ballot paper as the incumbent leader.

But some in the party have argued that under party rules he will need the nominations of 50 MPs and MEPs in which case he could struggle to get the necessary support.

Mr McCluskey insisted they were ready for any challenge. He said: "If anyone wants to change the Labour leadership, they must do it openly and democratically through an election, not through resignations and pointless posturing."

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".

"There are a lot of flat earthers out there who have got to come to terms with the fact the world is not as they like it," the source said.

However Mr Straw, the former foreign secretary, said Mr Corbyn's position was "wholly untenable".

He told Sky News: "It's probably the worst crisis since Labour's formation, in its current form, in 1918."

Former home secretary Lord Blunkett warned that Labour would be "annihilated" in a general election if Mr Corbyn remained leader.

"We don't want a third of Labour MPs - or even more - wiped out because Jeremy Corbyn would not be prepared to stand down and relinquish gracefully the leadership of the party to someone who can command the credibility," he told BBC News.

Ms 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 pressure on the Labour leader continued to mount throughout the day with further resignations, including shadow sports minister Clive Efford, shadow energy minister Alan Whitehead and shadow justice minister Andy Slaughter, who quit after turning down a promotion to the shadow cabinet.

Shadow communities and local government minister Liz McInnes also resigned from her frontbench role in a further blow to Mr Corbyn.