As many as 50 Labour backbenchers are thought ready to revolt - easily enough to wipe out the Prime Minister's Commons majority - when the issue comes before the House within weeks.

But Chief Whip Geoff Hoon insisted the Government could win the argument with rebel Labour MPs staunchly opposed to plans to hold terror suspects for up to 42 days without charge.

Mr Hoon was joined by Justice Secretary Jack Straw in maintaining that most Labour MPs were on the Government's side, while others could be won over with further reassurances.

The proposals came under renewed attack, however, from former attorney-general Lord Goldsmith.

In an article for The Sunday Telegraph, Lord Goldsmith said the measure undermined the very values the Government was trying to protect.

"Some supporters of 42 days say we have to take this step to protect our values and our way of life which terrorists threaten to destroy," he said. "But we start ourselves to destroy these values and the very basis of the free society which our ancestors fought hard to create if we readily give away critical liberties, such as the right we all have not to be arbitrarily held without charge."

Rebel Labour MP David Winnick said Lord Goldsmith's comments demonstrated the difficulties the Government would have getting the proposals past the Lords - even if they cleared the Commons. He said: "It seems to me that not only would the Lords say no on the first occasion should the Commons agree but it is quite likely they will persist in saying no."

But Mr Straw said that the "overwhelming majority" of Labour MPs supported the Government on 42 days, insisting Lord Goldsmith's criticisms were unjustified given the "protections" ministers were putting in place.

Mr Hoon told the BBC: "I'm confident that we can win that argument."