Tony Blair has issued a stark warning to Labour not to repeat the mistakes of the 1980s which consigned the party to 18 years in opposition.
In a rare intervention, the former prime minister said a shift to the left after the party's crushing general election defeat would be to treat voters as if they were "stupid".
His comments, to the centre-left Progress think-tank, came as the first public opinion poll in the Labour leadership contest put left winger Jeremy Corbyn on course for a shock victory.
Mr Blair derided the veteran backbencher as the "Tory preference" and said the party could not regain power if it was simply a "platform for protest" against cuts.
He mockingly suggested anyone whose "heart says I should really be with that politics" should "get a transplant".
Mr Corbyn dismissed claims that he would split the party and hit back at Mr Blair's suggestion that he was the "Tory preference".
"I would have thought he could manage something more serious than those very silly remarks," he said.
"Surely we should be talking about the situation facing Britain today, the situation facing many of the poorest people in this country today, and maybe think if our policies are relevant."
In a keynote speech setting out his economic policy, Mr Corbyn said austerity was a "political choice not an economic necessity".
He promised a "publicly led expansion and reconstruction of the economy" while protecting public services and increasing taxes on the wealthy.
With the unexpected surge in support for Mr Corbyn threatening to plunge Labour into turmoil, Mr Blair warned the party could not win on an "old- fashioned leftist platform".
He compared the position facing Labour to the 1980s when the party swung to the left under Michael Foot, paving the way for 18 years of Conservative rule.
"After the 1979 election the Labour Party persuaded itself of something absolutely extraordinary," Mr Blair said.
"Jim Callaghan had been prime minister and the Labour Party was put out of power by Margaret Thatcher and the Labour Party persuaded itself that the reason why the country had voted for Margaret Thatcher was because they wanted a really left-wing Labour Party.
"This is what I call the theory that the electorate is stupid, that somehow they haven't noticed that Margaret Thatcher was somewhat to the right of Jim Callaghan."
Mr Blair's intervention came as research by YouGov for The Times found Mr Corbyn was the first preference for 43% of party supporters - way ahead of bookies' favourite Andy Burnham on 26%.
Shadow home secretary Yvette Cooper was on 20% and Liz Kendall 11%.
The study also forecast that if Ms Kendall and Ms Cooper were eliminated and second preferences redistributed under the alternative vote system, Mr Corbyn would beat Mr Burnham by 53% to 47% in the final round.
The suggestion that Mr Corbyn - originally seen as the rank outsider - could turn the the tables and win was greeted with dismay among leading figures on the centre and right of the party.
Shadow education secretary Tristram Hunt warned that Labour could be reduced to a "pressure group" that would not have "broad reach into all parts of the United Kingdom".
John McTernan, a former special adviser to Mr Blair in Downing Street, turned on Labour MPs who had "lent" their nominations to Mr Corbyn to "broaden the debate", even though they did not want him as leader.
"They need their heads felt," he told BBC's Newsnight. "They are morons."