The House of Lords is expected to reject the government's plan to extend pre-charge detention for terror suspects to 42 days when peers vote later today.
Pressure is increasing on the government to abandon the controversial measure, but the Home Office insisted it was determined to give police the "powers they need to tackle terrorism".
Peers will today have their first chance to vote on the detention plan as the Counter Terrorism Bill is debated at committee stage in the second chamber.
It is widely expected that Labour, Conservative, Liberal Democrat and cross-bench peers will join forces to defeat the measure.
Independent peer and former West Midlands police chief Lord Dear will lead the assault with an amendment reading: "For the avoidance of doubt, nothing in this Act allows the secretary of state to extend the maximum period of pre-charge detention beyond 28 days."
His amendments are co-sponsored by shadow security minister Baroness Neville-Jones, Liberal Democrat justice spokesman Lord Thomas of Gresford and Labour's Baroness Mallalieu.
Prime minister Gordon Brown, speaking at a press conference on Monday morning, said: "The House of Lords should take the advice of the House of Commons on this matter."
MPs agreed to move the pre-charge detention limit from 28 to 42 days in June by only nine votes, after the Democratic Unionist Party's nine MPs supported the government.
Lord Goldsmith, the former attorney general, said the "pernicious provision" should be removed from the Bill.
Writing in the Guardian on Monday, he said: "I regard it as not only unnecessary but also counter-productive; and we should fight to protect the liberties the terrorists would take from us, not destroy them ourselves.
"This proposal is wrong in principle and dangerous in practice."
Former shadow home secretary and civil liberties campaigner David Davis predicted over the weekend that the move is "dead".
Forty-two British writers including Philip Pullman, Julian Barnes, Monica Ali and Ian Rankin each published a new piece of work lambasting the legislation.
And campaign group Amnesty International is holding a "mass sleepwalk" through the streets of Leeds in protest at the extension of the custody limit.
Speculation has been mounting that the government will not force the measure through using the Parliament Act should it be defeated by the Lords.
Peers are apparently prepared for a lengthy session of Parliamentary "ping pong", with the amended bill thrown back and forth between Houses.
Assuming Brown could continue to command a majority in the Commons on the issue, the process could end in deadlock.
The prime minister would then have to wait a year before using the Parliament Act to get the legislation onto the statute books.
But a Home Office spokesman said: "It is not true that the Counter Terrorism Bill is to be dropped.
"The government is determined to give the police the powers they need to tackle terrorism."
