Rebel Labour MPs have tabled a Commons amendment in a bid to thwart government plans to extend pre-charge detention for terror suspects to 42 days.
Among those to sign it are several backbenchers who previously backed a 90-day limit, suggesting the prime minister could lose a vote next month.
It comes amid reports Prime Minister Gordon Brown is preparing a compromise with MPs to avoid a damaging defeat.
He has previously said he would not back down and would win the argument.
But Chief Whip Geoff Hoon is seeking to broker a deal to prevent a Labour split, the Guardian newspaper reports.
It would involve greater Parliamentary oversight and the need by government to declare an "exceptional need" to use the power, the newspaper says, quoting government sources.
A defeat for Mr Brown would come as a major blow to his authority after Labour's local election drubbing and its climbdown over the 10p tax rate.
'Changed their minds'
Senior Labour backbencher David Winnick, who led opposition to the 90-day plan, which was defeated in 2005, has tabled the new amendment.
He said: "It does appear that some who supported the government in 2005 have now changed their minds and now take the view that the current 28-day limit is sufficient.
"I believe there is a reasonable chance that the government will be defeated, but I do accept that the government will do everything in its power to try to persuade some of my colleagues to agree on the basis of various concessions.
" I'm going to try to win this argument"
"If the government does not get its way, hopefully the issue will be closed for some time, unless there is compelling new evidence to show that it is necessary to go beyond 28 days."
In an interview with BBC political editor Nick Robinson on Thursday, Mr Brown insisted he would not compromise on the issue.
"I would rather win, and I'm going to put the argument... I'm going to try to win this argument," Mr Brown said.
"The reason is not because it's a sort of political game in the House of Commons. The reason is I'm concerned about the security of this country.
"We must make it possible for us to do everything in our power where there are terrorists, and particularly multiple terrorist plots, to take action."
But Frank Field - who led a backbench Labour rebellion on the abolition of the 10p tax rate - advised Mr Brown to stick to his guns as he believed the PM could win the vote.
Mr Field told GMTV, in an interview to be broadcast on Sunday: "He has a clear position on this. He should make the case again in the Commons."
Alternatives
On Wednesday, the Joint Select Committee on Human Rights, made up of MPs and peers, said an extended detention limit was "unnecessary".
It instead proposed a series of reforms, including ending the ban on granting bail in terror cases, and allowing post-charge questioning of suspects.
The committee has tabled several amendments to the bill calling for these changes.
It described the proposed safeguard against wrongful detention - a parliamentary vote after the 42-day power has been triggered - as "virtually meaningless".
Under government proposals, the home secretary could immediately extend the limit on pre-charge detention from 28 to 42 days, if supported by a chief constable and the director of public prosecutions.
MPs and the House of Lords would then vote to approve it within 30 days. If they rejected it, the extension would end at midnight on the day of the debate.
Attempts to extend it to 90 days in 2005 ended in Tony Blair's first Commons defeat as prime minister.
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