The BBC's Political Correspondent John Pienaar: "Frank Dobson is taking a gamble"

He told BBC One's Breakfast With Frost: "We thought I could carry on until the next election...but quite frankly, I can't do both jobs at once.

"It would be an insult to people working in the health service to have a part-time health secretary.

Dobson: Did not want to be part-time health minister

"Whenever the prime minister makes his Cabinet changes someone will succeed me as health secretary."

Mr Dobson's announcement expands the scope of the mini-reshuffle expected this week.

Five-way fight

He will now do battle with four other Labour candidates for selection as the party's official Mayoral candidate - former transport minister Glenda Jackson, left-wing MP Ken Livingstone, broadcaster Trevor Phillips and London minister Nick Raynsford.

Ken Livingstone: "He's clearly the strongest candidate produced so far"

Mr Dobson's formal entry into the race was thought to be a huge relief to Tony Blair, who is said to be anxious to prevent Mr Livingstone winning the nomination.

The other three challengers are not thought able to beat the Brent East MP in a straight ballot of London party members.