Former environment minister Michael Meacher has launched a bid to challenge Gordon Brown for the Labour leadership.
He promised a "centre-left" package featuring railway nationalisation, total withdrawal of troops from Iraq and a £7 an hour minimum wage.
Mr Meacher, who needs the nominations of 44 other Labour MPs to get onto the ballot paper after Tony Blair steps down, said he had 30 "signed up" and a further 25 indicating their support.
But Cabinet minister Jack Straw cast doubt on the credibility of his candidacy, telling reporters: "For those who want to place a wager, my advice is to find a horse."
And bookmakers seemed to share his assessment, making the 67-year-old veteran of Harold Wilson's administration a 50-1 outsider, with the Chancellor still the runaway favourite.
Insisting that Mr Brown's succession was not a "foregone conclusion", the Oldham West and Royton MP was flanked at the Westminster launch by leftist MPs Ian Gibson and Kelvin Hopkins.
But even Dr Gibson said he was not assured of his support, telling reporters: "He has asked some people if we would respect his decision to run, but that doesn't mean we will vote for him."
Mr Meacher unveiled a campaign platform of "peace, social justice, climate survival".
He launched a strong attack on Mr Blair's foreign policy, insisting Britain should no longer be the "glove puppet" of the US and vowing not to commit UK troops to any military action against Iran.
He floated a controversial proposal to link the wages of a company's best and worst paid staff, so that a salary hike for bosses would automatically increase workers' income.
