He told a packed meeting of local party members of his Holborn and St Pancras constituency in Camden: "I wanted you to hear first...my first public declaration that I have decided to seek the nomination to run as the Labour candidate for Mayor of London."

He has yet to officially declare his position - he is expected to do this some time next week.

At the meeting, Mr Dobson said he had lived and worked in the city for decades, and knew its "excitements, advantages and opportunities", as well as its "problems".

The cheering party raised the roof at Camden Town Hall

Mr Dobson said: "Our children, now grown up, went to local schools, used local youth clubs, played in local parks.

"We have all relied on local doctors, have been treated in local hospitals, experienced the growing failures of Tube and buses, seen much of the city get shabbier, the streets become more dangerous, more polluted, we have seen jobs disappear, and people reduced to living in cardboard boxes."

He added: "We know what is wrong with living and working in London and like everybody in the Labour party in London we want to do something about it.

"For my part, it's not how bad things are that drive me on, it's how much better things could and should be."

Health Secretary uncertainty

The announcement followed several months in which Mr Dobson insisted he wanted to stay in his current job and was not interested in running for mayor.

But will he stand down from the NHS?

It also followed fresh denials of reports that Prime Minister Tony Blair had pressurised Mr Dobson to ensure there would be a heavyweight contender who might prevent left-wing MP Ken Livingstone winning the nomination.

Opinion polls have consistently pointed to Mr Livingstone as Londoners' favoured choice to be mayor.

He is ahead of Tory candidate Lord Archer, Lib Dem candidate Susan Kramer.

He is also way ahead of the other prospective Labour mayoral candidates, MP Nick Raynsford, MP Glenda Jackson and broadcaster Trevor Phillips.

However, it is not clear whether Mr Dobson will stand down as Health Secretary while he contests the nomination for next May's elections.

He did not mention that in his Camden speech, and his aide declined to discuss the issue, saying: "Whatever happens is entirely a matter for the Prime Minister".

The aide continued: "It would be expected he would stand down if he was picked as the candidate."

PM Tony Blair is expected to carry out a Cabinet reshuffle next week.

Following Mr Dobson's announcement, his Tory shadow Dr Liam Fox said: "Now that Mr Dobson has decided to stand for Labour's nomination he must think very hard about his current role as Health Secretary."