Former attorney general Lord Goldsmith has attacked Government plans to hold terror suspects for 42 days without charge.
The peer, who was the Government's top lawyer until Gordon Brown became Prime Minister, warned the move would be an attack on the country's "fundamental freedoms".
His comments come just weeks before a crucial Commons vote, in which up to 50 Labour MPs are expected to defy the whip.
A widespread backbench rebellion could destroy the Government's majority and result in an embarrassing defeat for the beleaguered Prime Minister.
The Prime Minister and Home Secretary Jacqui Smith say police need more than the current 28-day limit to deal with increasingly sophisticated terror plots.
Lord Goldsmith said: "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.
"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: "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."
"There is a great deal of pressure on those likely to vote no in the Commons to change their minds."
But Justice Secretary Jack Straw said the "overwhelming majority" of Labour MPs supported the Government on the issue.
He added: "One of the things we are asking those who have reservations about these proposals is to cut through the rhetoric which surrounds the opposition to the proposals."
