John McDonnell has formally launched his campaign for the Labour leadership, saying the New Labour project is "over".
The Hayes and Harlington MP today pledged to fight Gordon Brown when Tony Blair steps down on a ticket that is "Labour - not old Labour or New Labour, just Labour".
The left-winger admitted he may find it difficult winning support for his challenge among Labour MPs - his manifesto includes reversing the use of the private sector in the NHS, rejecting nuclear power and repealing anti-trade union laws.
But Mr McDonnell insisted that grass roots Labour supporters were keen to see an end to Mr Blair's public service reforms and his foreign policy.
"I think the period of New Labour is over. It is not just the Blair leadership - and I think we need to let him go with dignity - but the whole thing is coming to an end. Support for New Labour has drained out of the party," he told politics.co.uk.
Mr McDonnell, who chairs the Socialist Campaign Group of Labour MPs, also dismissed other potential challenges for the leadership, saying they were all "avowedly New Labour" and were unlikely to win over party members or the trade unions.
Home secretary John Reid and education secretary Alan Johnson have both been named as possible candidates, and today former home secretary Charles Clarke suggested that Alan Milburn was also "leadership material".
The suggestion is likely to infuriate Mr Brown, who has had a long-running feud with the former health secretary, as will Mr Clarke's suggestion that the chancellor is no longer the inevitable successor to the Labour leadership.
"It depends very much on the circumstances of the election and Gordon setting out his views positively and setting a course for his leadership," he told London's Evening Standard.
"Certainly, he hasn't done that in the past. For a year I have urged him to set out a course openly. So has Tony. It is down to him now."
Speaking this afternoon, Mr McDonnell refused to criticise Mr Brown, saying he was "absolutely appalled" by the very public infighting among Labour MPs this week and would play no part in it.
"It was disgraceful and degrading for everyone concerned," he said.
"In all of that there wasn't a single issue of policy or principle raised. It's all about naked ambition from all sides... I say a plague on all their houses."
But he noted that all the other potential leadership candidates were "from the same stable", saying: "They all voted for the Iraq war, and for tuition fees, which completely alienated a generation of young people."