The Labour Party risks "annihilation" if Jeremy Corbyn wins the party's leadership contest, former prime minister Tony Blair has warned.
In an impassioned letter printed in the Guardian, Mr Blair said the party was walking "over the cliff's edge".
His comments come as Yvette Cooper is set to criticise Mr Corbyn, saying his policies are not "credible".
The Electoral Reform Society has said Labour should delay sending out ballots while checks are made on new members.
Labour said 610,000 were signed up to vote in the contest. It had 200,000 members before the general election.
The society is not involved in running the contest, but its deputy chief executive Darren Hughes said it was obvious there were some people who did not genuinely support Labour who had signed up to vote and should not be allowed to take part.
Voting for the contest starts on Friday and the result will be declared on 12 September.
Writing in the Guardian, Mr Blair said: "It doesn't matter whether you're on the left, right or centre of the party, whether you used to support me or hate me. But please understand the danger we are in.
"The party is walking eyes shut, arms outstretched over the cliff's edge to the jagged rocks below.
"This is not a moment to refrain from disturbing the serenity of the walk on the basis it causes 'disunity'.
"It is a moment for a rugby tackle if that were possible."
Last month Mr Blair, who won three elections and served as prime minister between 1997 and 2007, warned against the party moving further to the left and electing Mr Corbyn the party's new leader.
But in a further intensification of his warnings, he said: "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."
In her first explicit criticism of Mr Corbyn, fellow candidate Ms Cooper is to accuse him of "bad economics", saying he offers "old solutions to old problems".
Speaking in Manchester later, she will say: "I feel really strongly - not just as a leadership candidate but as a Labour Party member that desperately wants an effective Labour government - that his are the wrong answers for the future.
"And they aren't credible. That they won't change the world. They will keep us out of power and stop us changing the world."
While accepting that the comments could cost her votes, Ms Cooper is to also say there is a battle taking place for the soul of the party as she attacks her rival's policies on renationalisation, quitting Nato and quantitative easing.
Labour leadership contest
- Who are the candidates? Andy Burnham, Yvette Cooper, Jeremy Corbyn, Liz Kendall
- Dates: Ballot papers will be sent out on Friday; voting can take place by post or online. They must be returned by 10 September. The result is announced on 12 September
- Who can vote? All party members, registered supporters and affiliated supporters - including those joining via a union
- What is the voting system? The Alternative Vote system is being used so voters are asked to rank candidates in order of preference
- How does it work? If no candidate gets 50% of all votes cast, the candidate in fourth place is eliminated. Their second preference votes are then redistributed among the remaining three. If there is still no winner, the third place candidate is eliminated with their second preferences (or third in the case of votes transferred from the fourth place candidates) redistributed. It is then a head-to-head between the last two candidates
Numerous MPs have called for the leadership contest to be postponed over fears it has been infiltrated by supporters of other parties but Labour have rejected the claims.
The campaign teams of the other leadership candidates have now written a joint letter to the Labour party to complain about the lack of data they are receiving on people who have recently joined.
Candidates Andy Burnham, Liz Kendall and Ms Cooper have been told they will not be receiving lists of those who have recently signed up as members for another 10 days - although their details are being passed to the Electoral Reform Society.
In their letter they say this means there will not be a "level playing field" for all the candidates.
Campaign sources say they are concerned unions, which have declared their support for Mr Corbyn, will pass on the details of their affiliated supporters to his campaign, giving him an unfair advantage.
Mr Corbyn, a backbencher who was seen as a rank outsider at the start of the campaign, has been packing out meeting halls across the UK, with many of his supporters joining the Labour Party to vote for him.
On Tuesday the YouGov poll for The Times of 1,400 eligible voters put Mr Corbyn on 53%, 32 points ahead of Andy Burnham.
Mr Corbyn urged caution in response to the polls, saying while his campaign was "going very well", ballot papers had not yet been sent out.
Labour MP John Mann said the leadership race should be made into a "head-to-head" contest.
He told BBC Newsnight that MPs should select just one of the other contenders to go up against Mr Corbyn, engage in a televised debate and scrutinise what he was really proposing.