Frank Field provoked anger among Cabinet ministers by saying he would be "very surprised" if the Prime Minister lasted another two years in the job.

The former welfare reform minister said the premier was visibly unhappy in his position, describing it as a "tragedy". He also held out the prospect of a House of Commons defeat for the Government on the Budget without further reassurances on tax reforms.

His comments came as Mr Brown was trying to regain the political initiative after a welter of damaging disclosures by senior Labour figures and predictions of a by-election defeat in Crewe and Nantwich.

His 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 the premier was confident he would lead Labour into the next election, the spokesman added: "Of course he is."

Having promised fundamental reform of social care, the Prime Minister is set to outline his legislative programme for 2008/9 in a major announcement on Wednesday.

Mr Field, a long-time foe of Mr Brown and who has led the backbench revolt over the abolition of the 10p income tax rate, predicted the next election would be held at the latest possible opportunity, in 2010.

"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.