Blair and Dobson: The first joint appearance of the campaign

But he refused to repeat his sharpest past criticisms of former Labour MP Mr Livingstone, who he had described as extreme and "disastrous" for the capital.

Mr Blair said: "I am not going back into the business of attacking Ken Livingstone - everyone knows what I think of him."

However, he also said he would work with Mr Livingstone - who is standing as an independent - if he was elected: "I will work with whoever is the mayor. I abide by the democratic decision by the people of London, that is my duty as prime minister and of course I will carry it out."

Transport the key issue

It was the Labour leader's first appearance with Mr Dobson since before he won the party's controversial selection contest to become its mayoral candidate.

The delay sparked speculation Mr Blair had written off Mr Dobson's chances - polls suggest he is trailing Mr Livingstone by more than 40 points - and so did not wish to back a doomed campaign.

The prime minister said he had known Mr Dobson for almost 20 years, and that he could be relied on to stand up to the government if he had to.

He added: "His integrity, his courage and his honesty give him the qualities to do the job."

Mr Blair said "dividing lines" between the parties on the future of London's transport system were very clear.

He said Tory plans to privatise London Underground Tube were "madness" and that Mr Livingstone's plan for congestion charges would cost motorists �100 a month and delay Tube modernisation.

He said: "The Livingstone proposals are irresponsible on grounds of costs, particularly on congestion charging."

'Read the small print'

Ken Livingstone: Welcomed Blair's comments

Mr Dobson, who favours the government's plans for a "public private partnership" - partial sell-off - of the Tube, renewed his attack on Mr Livingstone's plan to finance improvements by raising bonds.

He said: "He [Mr Livingstone] talks about bonds as if they were premium bonds - they are not."

He added: "If Londoners go it alone and raise bonds as Ken Livingstone wants, Londoners will carry the risk."

Mr Dobson said Mr Livingstone's planned congestion charges would create "Checkpoint Charlies" across the city, with cars stopped and traffic made worse.

"Read the small print before you cast your vote ... if you change your mind afterwards you won't be able to change your vote. With Livingstone it will be a case of vote now and pay later."

Answering questions, Mr Blair said the "greatest problem" facing the Dobson campaign was "making sure the issues are focused on policy".

Addressing the media, he said: "His problem is getting you guys to focus on policy because when you focus on policy they will vote for him, when they focus on process or what happened a few months ago then the issues get obscured."

Livingstone welcomes move

Mr Livingstone later said that Mr Blair's comments amounted to a step nearer towards his own position: "By his comments today Tony Blair has moved some way to accepting that if I am elected the partial privatisation of the Tube will have to be reconsidered."

He said it was up to Londoners to decide the future of the Tube and that every opinion poll showed they "are totally opposed to the government's plans to break up and partially privatise the underground".

"No genuinely independent study has so far vindicated the government's plans. If Londoners elect a mayor who is opposed to the planned break-up of the Tube then the government will clearly have to reconsider."