In ferocious Commons question time exchanges, Mr Cameron said "This is a sub-prime deal from a sub-Prime Minister."

And he attacked Gordon Brown for flying to China with one of the principal bidders for Northern Rock (NR), Sir Richard Branson.

The Prime Minister hit back, accusing the Opposition leader of "opportunism" and putting at risk the stability of the economy.

He said he had no apology to make for travelling with Sir Richard, insisting: "It's not bad judgment to take British businessmen and women to win orders for British exporters."

Mr Cameron said taxpayers had a right to know what their total exposure was under the Government's plans for the bank.

"Let's be clear this is as much a rescue package for your reputation," he said. "So if the bonds aren't paid back and NR fails to meet its obligations, what is the total exposure? How much?"

Mr Brown told him: "The loans and the bonds are secured against the assets of NR. NR has a high quality loan book as everybody understands. It is our intention to get the taxpayer the best deal and to get the taxpayer their money back and to make profits."

Mr Cameron said: "You won't tell us how much the taxpayer is in for. It is £55 billion. You have effectively lumped every household in this country with a second mortgage. Taxpayers want to know how long it is going to be before they are off the hook. So how many years will it be before the bonds are repaid?"

Mr Brown said: "We would be the first to be repaid under the scheme. The loans and the bonds are secured against the assets of NR."