In a video message to voters in Barrow and Furness, John Woodcock said: "I am intending to seek renomination from my local Labour and Co-operative parties to be their official candidate, but I will not countenance ever voting to make Jeremy Corbyn Britain's Prime Minister".
The Labour rank-and-file are enamoured of the 67-year-old, thrice-married Corbyn, who has been described as everything from radical left-wing to Bolshevik and has often courted controversy.
Already a number of Labour lawmakers have announced they won't stand for re-election.
Labour leader Jeremy Corbyn.
Polling site Electoral Calculus, which surveyed more than 10,000 people last month, predicts that the Conservatives will increase their seats in parliament from 330 to 381, and that Labour will drop from 229 seats to 182.
The same analysis projects a 76 per cent chance of a Conservative parliamentary majority, and just a 5 per cent chance of a Labour majority.
She said: "It is unlikely, if I look at the figures, that we will have a Labour government". In contrast, 49 per cent opted for Mrs May.
After making a late entrance due to delays on his train back from a visit to Birmingham, Mr Corbyn won brief but polite applause for his address to MPs. "It's in a position where it is too strong to collapse and too weak to win".
Yet Mr Stewart - who has become a father for the second time following the birth of baby son William six days ago - believes the election is a positive move.
"I'm not going to be calling a snap election".
Given that Labour secured 99 fewer seats than the Tories in 2015, when the polls were much closer, the scale of the challenge facing Mr Corbyn is clear.
Mr Corbyn is being nearly universally regarded as "unelectable", and Mrs May was surely looking at his struggles and anticipating an opportunity to strengthen her majority when she made her decision this week. The country needs a government that will rebuild our mismanaged economy and invest in the NHS, schools, and social care, reversing damaging cuts. About 100,000 new members joined the party in the two weeks following last June's Brexit referendum. That would leave the government with little more than a week to rid the bill of troublesome amendments inserted by the Lords and have it passed in the "wash up" - in which all legislation now in play is dealt with quickly before a dissolution.
"It's also a cultural schism. He's an activist, he's not really a public leader", Dr McAngus said.
But Mr Corbyn told MPs Mrs May could not be trusted, after u-turning on her promise not to call an early election and accused her of running scared of TV debates during the campaign.
For one, his critics within the party have not been able to find an alternative prospective leader to rally behind.
The letter continues: "We understand that, in the immediate run-up to an election, signalling a willingness to work with other parties might be hard but we hope you'll agree that the times we are living in require leaders to be courageous and visionary, to actively build a more positive politics".
"But he would probably step down", Dr McAngus said. "It's just that he's an activist, he's not a political party leader".