Tony Blair has launched another attack on Jeremy Corbyn's bid for the Labour leadership, warning the party faces "annihilation".
Writing in The Guardian on Wednesday evening, the former prime minister said the Labour party was in a danger "more mortal today than at any point in the over 100 years of its existence".
Earlier today the party announced over 600,000 people had registered to vote for Ed Miliband's successor. Corbyn, the veteran leftwinger, is now seen as the overwhelming favourite to win.
His rivals, Andy Burnham, Yvette Cooper and Liz Kendall have also written to the party to complain that the voting system may not be "a level playing field" - a charge Corbyn's campaign has rejected.
Blair has attacked Corbyn's politics once before. But in his intervention today the ex-Labour leader goes further in recognition that Corbyn is winning the contest. "The party is walking eyes shut, arms outstretched, over the cliff’s edge to the jagged rocks below," he said.
"The leadership election has turned into something far more significant than who is the next leader. It is now about whether Labour remains a party of government.
"If Jeremy Corbyn becomes leader it won’t be a defeat like 1983 or 2015 at the next election. It will mean rout, possibly annihilation. If he wins the leadership, the public will at first be amused, bemused and even intrigued but ... the public mood will turn to anger."
Blair added: "Jeremy Corbyn doesn’t offer anything new. This is literally the most laughable of all the propositions advanced by his camp."
Corbyn's campaign told The Huffington Post in response to Blair's comments: "We are keeping our campaign positive and remain focussed on our policies which offer the sound economic choice of investment and growth, not the politically driven agenda of austerity and cuts preventing economic recovery."
Jeremy Corbyn's London rally
Labour revealed this evening that in total 610,753 people applied to vote in its leadership election, with around 400,000 people having signed up since the election.
The party indicated some may still be denied a vote if they are found to be Labour opponents trying to unfairly tip the contest in one candidate's favour.
On Wednesday, Labour MP Simon Danczuk said Corbyn would face a coup against him by his MPs on the first day after he was elected leader.
Corbyn, the veteran leftwinger, has become the surprise frontrunner in the race to succeed Ed Miliband - much to the horror of those in the centre and on the right of the party.
Danczuk, who has called on the contest to be halted, said Corbyn would not last in office for "about 12 months". Asked on LBC radio if the plotting would begin on "day one", he said: "Yeah, if not before. As soon as the result comes out. People in the PLP [Parliamentary Labour Party] aren't going to put up with it. "
The Rochdale MP, who was also a harsh critic of Ed Miliband, said he would not put up with the "crazy leftwing policies" advocated by Corbyn.
Labour had to extend the deadline for people to register to vote in the leadership contest from midday until 3pm today after its website crashed. Ballot papers will be sent out on Friday and the result will be announced on September 12.
Commenting at the end of registration to participate in the leadership election, Corbyn said today: "The response to the Labour leadership race from party members and supporters, longstanding and new, is proof of the optimism in the possibility of a popular, modern alternative to the Conservative government.
"All of our focus must be on channelling this extraordinary movement towards a united party that is able to find its confidence and defeat the Conservatives in 2020.
"I particularly want to pay tribute to the Labour Party’s staff who have worked so hard to deliver a robust selection system in totally new circumstances. The professionalism and commitment of the party’s staff shows our movement at its best."
The party has seen a surge in the number of people able to vote in its leadership contest since the election.
However some in the party are worried that it is being infiltrated by the hard left who are actually opposed to Labour values.
Around 1,200 members or supporters of another party have already been excluded from voting in the leadership contest, including hundreds who stood against Labour in local or national elections.
They included 214 from the Green Party, 37 from the Trade Unionist and Socialist Coalition, 13 Tories, seven from Ukip and one from the BNP.
On Tuesday a YouGov poll for The Times showed Corbyn now has a staggering 32-point lead over Andy Burnham, his nearest rival.
The left-wing candidate is poised to win in the first round of voting having polled 53 per cent of the vote - a 10 per cent surge on a month ago. Corbyn, who wants Trident scrapped and industries re-nationalised, has pulled well ahead of Burnham on 21 per cent, Yvette Cooper on 18 per cent of the vote, and Liz Kendall with just eight per cent.
YouGov President Peter Kellner has said he would be "astonished" if Corbyn did not win the leadership race but added: "I have seldom released a poll with as much trepidation as I have done this time."