He told an audience of London Labour Party members on Wednesday that he wanted them to concentrate on policy and not the process.

The prime minister's words are being seen as a clear endorsement for Mr Dobson, who is running against Ken Livingstone and Glenda Jackson to become Labour's candidate.

Mr Blair said that "the reason I believe that Frank Dobson will be a good person to be the mayor of London is that I believe he would do a good job on the policies that matter for the people of London".

Although the prime minister did not name another prospective mayoral candidate, Mr Livingstone, a former leader of the GLC, he said he did not want to see a return to the 1980s when the party lost its way.

Frank Dobson would do a good job on policies, said Mr Blair

Mr Blair said: "This is nothing to do with me being arrogant or a control freak.

"We are in fact giving away power when we decentralise government in London.

"But I never want the Labour Party to go back to the early 1980s, to become a by-word for extremism or end up with Conservative MPs in parts of London because the Labour Party lost its way and ceased to be a force for good decent Labour values. I am never going back to those days."

Mr Blair was speaking on the same day as Mr Livingstone placed an advertisement in a London newspaper declaring he was better placed than Mr Dobson to win the mayoral election in May.

The advert pictures Mr Livingstone alongside the former health secretary and said: "Who do you think Jeffrey Archer would rather run against?"

It continued: "According to the latest poll in this newspaper, Ken Livingstone is comfortably ahead. If he were the Labour candidate, he'd win a handsome 63% of the total share of the vote (to Jeffrey Archer's 27%)."

It went on to warn that if Mr Dobson was picked, another poll suggested that Jeffrey Archer "would not only close the gap but actually overturn it".

'Clear and honest choice'

Commenting on Mr Livingstone's tactics, Mr Blair said he did not think it was the type of advertisement the party should have.

Labour Party members, he said, can make up their own minds - but he stressed that they should concentrate on policy.

Later, Mr Livingstone said he welcomed Mr Blair's suggestion that the contest should now concentrate on policies not personalities.

He said: "I am the only Labour candidate standing on the platform that I will oppose passing control over any part of London Underground to Railtrack.

"That presents the London Labour party with a clear and honest choice."

Front-runner

Mr Livingstone refused to be drawn on whether Mr Blair had intervened because polls had shown he was the clear front-runner, far ahead of Mr Dobson, in the race to be Labour's candidate.

Mr Dobson has been seen as Downing Street's favourite to become the Labour candidate.

On Wednesday, Mr Dobson told London business leaders the mayor of London would not be a return to the GLC.