Victory for Jeremy Corbyn in the Labour leadership race would be a "car crash", Tony Blair's former spin doctor has warned.
In a strongly-worded intervention Alastair Campbell called for Labour supporters to sign up to vote for "anyone but Corbyn", claiming that the leadership contest was now a battle to save the party.
A triumph for Mr Corbyn would show that Labour had "given up on being a serious party of government", Mr Campbell said.
The key New Labour figure said just as if he was on a bike ride and saw a car crash about to happen he would alert the drivers to the danger "so I think I have to say something about what appears to be happening to Labour right now".
It was a "car crash, and more", he said.
He welcomed the fact that Mr Corbyn had inspired people to show an interest in politics but advised those who saw him as "some kind of cross between Russell Brand, Nicola Sturgeon and their favourite uncle" to examine Labour's recent history.
Writing on his blog Mr Campbell said: "The two main parties, when choosing a leader, are picking the person they intend thereafter to try to persuade the people of the UK 'this is who should be your Prime Minister.'
"And yet the Labour Party, if it elects Jeremy Corbyn as leader, is selecting someone that every piece of political intelligence, experience and analysis tells you will never be elected Prime Minister."
He added: "Whatever the niceness and the current warm glow, Corbyn will be a leader of the hard left, for the hard left, and espousing both general politics and specific positions that the public just are not going to accept in many of the seats that Labour is going to have to win to get back in power."
Mr Campbell backed former Cabinet minister Alan Johnson in claiming that "the madness of flirting with the idea of Corbyn as leader has to stop".
He said: "That means no first preferences, no second preferences, no any preferences. It frankly means ABC, Anyone But Corbyn."
In a blunt message to Mr Corbyn's rivals Andy Burnham, Yvette Cooper and Liz Kendall, Mr Campbell said they "need to show now that they understand they are in a fight not just to be Labour leader, but to save the party".
Ms Kendall, regarded as the Blairite candidate in the field, told BBC Radio 4's World at One: "I don't think Jeremy's policies are right for 2015, let alone 2020 or 2025."
Asked if she was the "heir to Blair", Ms Kendall said: "I don't think so, actually. I have always looked at the history of the Labour Party and how we won our great election victories in 1945, 1964 and 1997 and what struck me about all of those was that our values always remained the same but we really applied them to the world as it is and to the future, not the world as we wish it would be."
There were further signs of concern within Labour at the leadership election process, which has been hit by claims that members of far-left groups and Conservative supporters had signed up to take part.
It is thought that around 190,000 of the 390,000 people who are eligible to vote in the leadership election have signed up since the party's general election defeat in May.
They include affiliated supporters who - under new rules adopted under Ed Miliband - have to pay just £3 to take part as well as individual members of trade unions and other organisations which are affiliated to the party.
Labour MP Barry Sheerman, a supporter of Ms Kendall, told the BBC: "We know there are a large number of Green candidates registered to vote - I'm not so much worried about those but people from the Socialist Workers Party, people from the Conservative Party.
"There are all sorts of people from up and down the country joining the party for what reason? We don't know - some of those reasons I think are malign.
"I have stood up to people who have tried to enter our party over the years and take it over. We have always been successful in keeping those out because we are democratic socialists, we are not revolutionary socialists."
But Labour MP Diane Abbott, a prominent backer of Mr Corbyn, said: "I think this is ridiculous. It's coming from people who think their side is going to lose. It would be absurd if you just halted an election because you're worried your side is going to lose."
Labour insists that it has "robust" systems in place to prevent "fraudulent or malicious" applications.