More than 600,000 people are set to choose the next Labour leader amid warnings from Tony Blair that the party risks "annihilation" under Jeremy Corbyn.
The former prime minister warned Labour could fail to be in government again and is in "danger more mortal" today than at any point in its existence, as veteran left-winger Mr Corbyn continues to remain the frontrunner in the contest.
Mr Blair directed his remarks at long-standing members and called on them to "save the party" and heed his warnings regardless of whether they used to "support me or hate me".
He added voters would punish the party as they would view themselves as "victims" of the Conservative-led Government's policies and Labour's self-indulgence".
Writing for The Guardian, Mr Blair said: "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."
Labour MP Diane Abbott, a supporter of Mr Corbyn, suggested Mr Blair's comments "might be counter-productive", while deputy leader hopeful Angela Eagle cautioned: "Using apocalyptic language isn't very helpful."
She also told BBC Two's Newsnight: "Let's just trust our members to actually vote for the person that's going to lead our party."
Mr Blair's latest intervention came as Labour confirmed the possible total electorate for the leadership vote is 610,753 after the deadline for registering passed.
Party data shows there are currently 299,755 members, 189,703 people from bodies affiliated to the party and 121,295 people who have paid £3 to register as a supporter in order to have their say.
As the deadline approached, Graham Stringer joined three other Labour MPs in calling for the race to be halted while the party attempts to weed out bogus anti-Labour voters who have signed up as registered supporters.
The surprise emergence of Mr Corbyn as the apparent frontrunner in the race has led to calls for it to be re-run over allegations that it has been exploited by hard-left groups and political opponents including Conservative activists.
But Mr Corbyn - who is the bookies' favourite ahead of rivals Andy Burnham, Yvette Cooper and Liz Kendall - said the public response to the leadership contest was proof of optimism that a popular, modern Labour Party could defeat the Tories.
Labour said it has already excluded 1,200 members or supporters of another party and will continue to verify those signed up and remove people who are not entitled to a vote.
But Mr Stringer warned legal action could be taken against the party unless it pauses the process, joining colleagues Barry Sheerman, John Mann and Simon Danczuk.
The GMB union said the total number of its members recorded with the Labour Party as affiliated supporters was 23,840.
No other union has given details of how many of their members have registered to vote.
The figures came as all three of Mr Corbyn's rivals called for quicker access to information about the latest raft of people who have been signing up to vote.
In a letter to Labour Party chiefs, it is understood they have raised concerns that the information has not been made available to them despite being ready to send to the printers to prepare ballot papers.
Mr Mann has also suggested Labour MPs should informally renominate and choose their preferred candidate from Mr Burnham, Ms Kendall and Ms Cooper to challenge Mr Corbyn in a head-to-head contest.
The Bassetlaw MP told Newsnight: "Let's have a proper head-to-head, including a televised debate, which would have a huge amount of interest, and then Jeremy Corbyn can be put under some proper scrutiny with actual debates."