Left-wing backbencher John McDonnell has said he will run for the Labour leadership when Tony Blair stands down.
John Mcdonnell launches his challenge in Westminster
The Hayes and Harlington MP said he was not a "stalking horse" candidate trying to topple Tony Blair.
But he wanted to ensure there was no "coronation" for Gordon Brown once Mr Blair leaves Downing Street.
Mr McDonnell said he wanted to repair the damage done by New Labour and represent people "who no longer feel they have a voice".
He promised to tour the country with his campaign and urged lapsed Labour members to rejoin the party to get involved in the debate.
"This is not a move against Tony Blair - it's a move against New Labour and the way it's formed at the moment," he told reporters on Westminster's College Green.
"We've had no debate. We in the party have felt very disenfranchised."
The 54-year-old is chairman of the left-wing Campaign Group has been one of party's most rebellious MPs.
He promised to "get back to grass roots campaigning" and end the spin of New Labour.
He predicted Mr Blair, who says he will leave before the next election, would announce his resignation in the next 12 or 18 months.
"Some have argued that instead of an open democratic election for the leader of the party, there should be a smooth transition or virtual coronation of his successor," said Mr McDonnell.
"This would deny party members the opportunity of openly debating the issues facing our party and the future direction of the party."
He said a handover to Mr Brown could soon be followed by a "smooth transition" to Conservative leader David Cameron.
He accused the government of systematically alienating sections of Labour supporters - students, pensioners, public service workers, trade unionists, green activists and civil liberties and peace campaigners.
"This is reflected in lost votes, lost elections, lost members and a Labour prime minister having to rely upon Conservative votes in Parliament to force through legislation," he argued.
But Mr McDonnell said he was happy to let Mr Blair stand down at a time of his choosing rather than try to hasten his departure.
"We have a real tradition in the Labour Party of loyalty to the leadership," he said. "We don't assassinate the leadership."
He insisted he was not a "stalking horse" but a "serious challenger" for the Labour leadership.
He denied his announcement would destabilise Labour and said it was not prompted by the recent "cash for peerages" allegations.
He said he had been wanting to announce his candidate for weeks but his plans were foiled every time by a new government controversy.
"We can't keep on delaying," he added.
Mr McDonnell has voted against Labour in the Commons on several occasions.
He has spent 25 years in politics and was the deputy leader of the Greater London Council under Ken Livingstone.