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Gordon Brown has "tempers of an indescribable nature" according to the rebel backbencher Frank Field, who has become the first Labour MP to predict that the Prime Minister will not be in place at the next election.

In an astonishingly outspoken attack, Mr Field said that the public could see Mr Brown was unhappy and asserted the election would be held on the last possible date in 2010.

This prompted immediate denials from Downing Street, who insisted Mr Brown would still be in place at the next election.

Mr Brown’s spokesman said: "The Prime Minister’s general view on this is that he is not going to be distracted by this sort of stuff. What the Prime Minister is focusing on is the business of government and the big issues that are facing the country.

Asked whether Mr Brown was confident that he would lead Labour into the next election, the spokesman added: "Of course he is."

This came as the Prime Minister attempted to launch a fightback, with a major speech at the King's Fund on social care. Mr Brown is seeking to find ways of preventing the elderly being forced to sell their homes to finance time in care homes. However, the government is not putting forward any concrete suggestions at this stage.

Speaking on the BBC World Service last night, Mr Field - a noted opponent of Mr Brown who clashed with him as a minister in 1997 - said that his personality, rather than the recent memoirs by Cherie Blair, John Prescott and Lord Levy, was causing real difficulty.

"The awful fact that is coming across is that he seems so unhappy in himself. And I think everybody in the country who has ever watched a news clip of the Prime Minister realises that, and it's a mega problem for him and the government," he said.

"That is clearly part of the tragedy on a personal level, as well as on a party, government and country level, that someone whose real aim in life was to be Prime Minister now has the task and seems to be so lacking in enjoyment in trying to try it out."

Mr Field said he needed no persuading that the Prime Minister could be “testy” and “go off like a bloody volcano”, as described by John Prescott, the former deputy prime minister. The former welfare reform minister described “tempers of indescribable nature” in which Mr Brown would shout “in a rage”.

He became the first Labour MP to publicly question whether Mr Brown will be in place at the next election.

"I'm sure the next election will be two years two weeks away. That is the very last moment that we can legitimately call an election. And I will be very surprised if he is still leading the Labour Party then and therefore leading us into the election campaign.

"If he asked my advice, I would say to him talk to the people who you love most and who love you, and act on their advice," Mr Field added, "because certainly with this Budget coming up with the 10p cut in the first rate of tax...if we don't get a deal, I think there is enough anger on the Labour backbenches who will with others block the Budget going through and that will make his position intolerable."

He said the British public could see what was wrong. "The Prime Minister looks so unhappy within his own body, and it conveys the most dismal message to people."

Ministers loyal to Mr Brown today hit back at Mr Field. Ed Balls, one of the Prime Minister's key economic advisers when he was Chancellor under Tony Blair's governments, retorted that he would be "surprised if the Prime Minister will be taking Frank Field’s advice generally".

Pressed at a press conference on whether Mr Field was on a "one man mission to take down Mr Brown", he replied pointedly: "I’ll leave you to draw your own conclusions from what was said this morning."

Mr Balls added: "We have all known Frank for many years, and I think he used to work on his own when he was in opposition. He used to work on his own on the backbenches. I think he used to work on his own when he was a minister as well."

Mr Balls said he was convinced Mr Field would now reject attempts by the Prime Minister and Chancellor Alistair Darling to hammer out a compromise over the explosive 10p tax issue.

"I think it is very unlikely on the basis of what I have seen that Frank Field will support any proposals that are brought forward by Government on the 10p tax."

Alan Johnson, the Health Secretary, said that the "character assassination" of Gordon Brown should stop. "Frank Field doesn't get on with Gordon Brown - well we've known that for 10 years," he said.

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