The prime minister has welcomed Ken Livingstone back into the Labour Party after nearly four years in exile.
Mr Livingstone was expelled for running as an independent in 2000
The London mayor was kicked out for standing as an independent after failing to be selected as Labour's official candidate in 2000.
Tony Blair admitted his prediction that Livingstone would be a "disaster" for London had been proved wrong.
Mr Blair said: "I think I should be big enough to say the prediction I made...has not turned out to be right."
The decision to readmit Mr Livingstone came after he was interviewed by a five-member panel of senior Labour figures.
In what was being dubbed a "loyalty test", the mayor had to show he was ready to stick by party rules.
Nicky Gavron last month stepped aside as Labour's mayoral candidate and is now expected to run as Mr Livingstone's deputy.
After a meeting of Labour's National Executive Committee rubber stamped his return, the mayor told BBC London: "I'm delighted because the reality is we will get a better hearing for London in the government.
"I suspect this will help on a lot of projects where we have been pressing for government funding and backing and I think we will see some results of this within days, not weeks."
He had never "curbed his tongue" and nobody was asking him to do that now, he said.
Mr Livingstone argued there had been no point dragging out his expulsion.
"You basically can be out after five years on a murder rap and that does seem a little bit much for standing for an election," he added.
And Mr Blair conceded that despite his earlier fears, Mr Livingstone had done "a pretty good job" running the capital.
"My prediction that he would be a disaster has turned out to be wrong and I think when that happens in politics you should just be open about it," he told BBC London.
"I thought that on crime and business and transport he would be bad for London - actually on crime he's increased police numbers and cut crime.
"On transport, the congestion charge has basically worked ... he is working well on business.
"If the facts change you should be big enough in politics to say your mind changes."
Mr Blair said that his comments four years ago were made without the benefit of Mr Livingstone's record.
"If I thought he was bad for London today, I would not back him, no matter what was the situation, but I actually can't say, hand on heart, he's been a disaster for London.
"He isn't. He's actually done a pretty good job."
Mark Seddon, a member of Labour's National Executive Committee, said Mr Livingstone had "essentially supported most government policies" but not the part privatisation of the London Underground.
Senior figures such as Chancellor Gordon Brown and Education Secretary Charles Clarke were thought to be unhappy about Mr Livingstone's return.
Former Labour leader Neil Kinnock had said he was "irretrievably" opposed to the idea.
'Marriage of convenience'
Liberal Democrat mayoral candidate Simon Hughes said polls suggested Mr Livingstone would have been better continuing as an independent.
The decision was a "marriage of convenience", he told BBC News 24.
"It's a desperate attempt by Labour, who had a candidate until a couple of weeks ago, to find somebody to whom they can pin their hopes and aspirations and hope to scramble home in June because they are the last big elections before the general election."
Conservative Steve Norris accused Mr Blair of a cynical move to stave off humiliation in the election.
"It suits Ken, seems to suit Mr Blair but one lot of people it does not suit, I think, is Londoners," he said.
Green Party candidate Darren Johnson said: "Loyalty to London is the real test, not loyalty to Blairism.
"For those who voted Livingstone last time but can't stomach voting Labour, the Green Party is the only serious choice."
Mr Livingstone now faces a poll of London Labour members over whether they want him as the mayoral candidate.