Britain is still in danger of losing its triple-A credit rating and the Conservatives would make protecting it a priority if elected next year, Shadow Chancellor George Osborne said.

In an interview with Reuters yesterday, Mr Osborne also said most of the Conservatives' deficit reduction measures would come from spending restraint.

Chancellor Alistair Darling announced tax rises and a levy on bank bonuses to help plug the government's gaping deficit, but there was little in the way of spending cuts, raising concerns about the near-term debt outlook.

Asked if Wednesday's pre-budget report measures were enough to soothe fears that Britain might lose its triple-A credit rating, Mr Osborne said: "No, I don't think they are."

"I don't think it is a credible plan. Unfortunately, the measures announced yesterday don't start tackling the deficit until 2014/15 and that is far too late. "The thing I'm aiming for is making sure that Britain keeps its credit rating."

Ratings agencies said the outlook for Britain's sovereign debt had not been materially changed by the PBR and they are waiting to see what measures a future government will take after next year's election, which the Labour party is expected to lose.

The Conservatives have said they will hold an emergency budget within 50 days of an election if they win. Their focus will be on spending cuts.

"The bulk of dealing with the deficit has to come from spending restraint," Mr Osborne said.

But while avoiding the government's further increase in national insurance contributions was a "key priority," Mr Osborne said, he could not promise they would revoke it.

Bank of England Governor Mervyn King has urged the government to come up with a credible plan for reducing the budget deficit, although he has said the timing must depend on the state of the economy.

Mr Osborne said the government should follow Mr King's advice to tackle the deficit to help keep borrowing costs down. And he said that although monetary policy was a matter for the central bank, it was important to keep interest rates low.

"We've got to keep interest rates as low as possible for as long as possible."

He also ruled out imposing a windfall tax on banks but said further measures could be taken to rein in their risky behaviour, citing the IMF's idea of imposing an insurance levy on institutions as something that should be looked at.