Tony Blair – one of Labour's most successful leaders – has intervened in the party's leadership contest by urging supporters to reject a shift to the left.
The speech came after a shock poll put left-wing Jeremy Corbyn 17 points ahead in the leadership race.
The 62-year-old, making a rare speech on British politics to the think tank Progress, said he would not want to win on an old-fashioned leftist platform" and pressed the party to "move on".
"But don't for heaven's sake move back. If we do, then the public won't vote for us – not because our thoughts are too pure, but because our thoughts are out of touch with the world they live in," he argued.
Blair added: "We misunderstand the difference between radical leftism, which is often reactionary and radical social democracy."
However, the former prime minister said that Labour could win another general election but only if the party uses its values as its guide, rather than "our distraction".
Blair also claimed that the electorate had voted for the Tories out of fear at the election, a fear of "getting a Labour party that didn't get what the modern world is about".
The comment was a clear criticism of Ed Miliband's leadership and Blair added salt to the Doncaster North MP's wounds by saying that "[Miliband] thought centre ground had shifted left, and I don't think it shifts in that way".
But Blair, who was in power between 1997 and 2007, refused to publicly endorse one of Labour's four leadership hopefuls, explaining that he thought such a move would be "possibly not even helpful".
The 66-year-old's politics stand in stark contrast to Blair. Corbyn is a vice chairman of the Campaign for Nuclear Disarmament (CND) campaign and campaigned against Britain's involvement in the 2003 Iraq war.
But the veteran parliamentarian seems to be popular within his party as 43% of Labour voters are set to select him in the leadership contest, according to YouGov.
The survey, which was commissioned by The Times, also put Andy Burnham on 26%, Yvette Cooper on 20% and Liz Kendall on 11% on first preference votes.
Bur Blair dismissed the Corbyn-mania, with a withering putdown: "People who say their heart is with Corbyn, get a transplant".
Corbyn is ahead in local party nominations, with 74 backing the socialist against 73 constituency parties picking Burnham, according to The New Statesman.