Tony Blair has warned of the "annihilation" of the Labour party if Jeremy Corbyn is elected as leader.
In a plea to voters published in the Guardian he says "even if you hate me, please don't take Labour over the cliff edge."
It comes as registration to vote in the leadership contest closes with 610,000 members able to participate.
Concerns have been expressed over the validity of the registrations with accusations of people trying to "infiltrate" the party.
He says the party is "is in danger more mortal today than at any point in the over 100 years of its existence."
“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.”
He believes the public will be amused, bemused and intrigued at first if Corbyn wins the leadership race but says the public mood will turn to anger in the absence of an effective opposition.
In a previous intervention in July, Blair advised those considering voting for Corbyn to "get a transplant".
The campaign offices of Corbyn's three rivals have today issued a joint letter expressing concern over the ballot to Labour HQ.
The offices of Liz Kendall, Andy Burnham and Yvette Cooper have also issued a joint letter regarding a lack access to voter data needed order to properly co-ordinate their campaign efforts.
A YouGov poll released yesterday suggested Corbyn had a 32 point lead over his fellow leadership candidates.