Allies of Gordon Brown today accused his critics of "putting the knife into" the Prime Minister, as a senior Labour MP predicted he would be ditched as the party's leader before the next election.
Former minister Frank Field, who has led the backbench revolt over the abolition of the 10p income tax rate, said he would be "very surprised" if Mr Brown was still in charge at the time of the General Election, expected in 2010.
The PM faces a backbench rebellion which will block the Budget and make his position "intolerable" unless he reassures MPs over the impact of the tax change on the lowest paid, said Mr Field.
The Birkenhead MP said the next election would almost certainly be at the latest possible opportunity, in two years' time.
"I would be very surprised if he's still the leader of the Labour Party then and therefore leading us into the election campaign," he told the BBC World Service.
Mr Field said the removal of the 10p rate, announced by Mr Brown in his last Budget as Chancellor in 2007 but coming into effect in this year's Finance Bill, had caused greater anger on the Labour backbenches than he had ever previously seen.
"If we don't get a satisfactory deal, I think there's enough members on the Labour backbenches who will, with others, block the Budget going through," he said.
"That will make his position intolerable at that stage."
His comments, following a welter of personal criticism of Mr Brown over the weekend, prompted Health Secretary Alan Johnson to acknowledge the "knives (are) coming out".
The exchange comes as Mr Brown tries to stage a fightback after damaging disclosures in the memoirs of Cherie Blair, John Prescott and Tony Blair's former fundraiser, Lord Levy.
Mr Johnson voiced frustration that the political agenda was being dominated by the "character assassination" of the Prime Minister.
"What the public are concerned about is now, all these knives coming out," he told BBC Radio 4's Today programme.
"I'm not a great Brown fan club leader but I respect him as a really, really decent, good, able politician. Is he perfect? No, he's not, nor is anybody else in the world.
"But can't we just set this true confessions stuff to one side?
"Some people see an opportunity to put the knife into somebody they don't like."
Mr Field said he needed no persuading that the Prime Minister could be "testy" and "go off like a bloody volcano", as described by Mr Prescott.
The former welfare reform minister described "tempers of indescribable nature" in which Mr Brown would shout "in a rage".
But Mr Field said people would not judge the PM on the memoirs that were now being published but on the way Mr Brown currently appeared to the public.
"The awful fact that's coming across is that he's so unhappy in himself," he said.
"I think everybody in the country who's watched a news clip with the Prime Minister on recognises that."
He said it was a "tragedy" given Mr Brown's lifelong ambition to reach Number 10.
"Somebody 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 carry it out," he went on.
Mr Brown was trying to reclaim the political initiative this week, setting out his draft Queen's Speech programme for the autumn with promises of new measures on schools and health.
It followed another round of disclosures about his turbulent relationship with Tony Blair in the form of Mr Prescott's autobiography, serialised in The Sunday Times.
The former deputy prime minister described Mr Brown as a "frustrating, annoying, bewildering and prickly" man who could "go off like a bloody volcano".
Mr Prescott's account came hard on the heels of the disclosure by Mr Blair's wife, Cherie, in her autobiography that Mr Blair would have stood down before the 2005 General Election if Mr Brown had been prepared to back his plans for city academies and foundation hospitals.
Meanwhile, another recent autobiographer, former Labour fundraiser Lord Levy, repeated his claim that Mr Brown must have known about the secret loans from wealthy party backers which led to the "cash for honours" police inquiry.
Aides dismissed the allegation as "complete, unsubstantiated garbage". Mr Brown has always insisted that as Chancellor he was careful to distance himself from party funding matters.Reuse content