Tony Blair has said he still backs plans to hold terror suspects for up to 90 days without charge.
He said he believed the evidence backed longer detention and that he had not changed his mind since losing a Commons vote on the issue last year.
But the prime minister, in an e-mail question and answer session, said he wanted to "proceed by consensus".
New anti-terror plans could be brought in before Christmas, he added, saying that police wanted longer detentions.
The proposals, he said, would be "based on an analysis now of what has gone on in the past few months and how we make sure we have the most effective laws to deal with the terrorist threat that we face".
He added: "The issue to do with the number of days of detention will be part of that."
The government's plans to bring in a 90-day limit on detentions were voted down by the Commons last year, with MPs and peers eventually settling on 28 days. Previously, the limit had been 14 days.
Mr Blair said: "I supported 90 days before on the basis that, particularly, the police handling terrorism for us thought that that was what was needed.
"But we have got to look at it again."
Home Secretary John Reid said on Wednesday that the UK faced a "wave" of terrorist plots, prepared strategically and directed from abroad by al-Qaeda.
Although no anti-terror bill was included in the Queen's Speech, pending Mr Reid's review of options, the government promised to "fill gaps" in legislation.
Lord Carlile, the government appointed expert who oversees terrorism laws, said on Wednesday that he expected fresh anti-terror plans to be published in the new year.
He told BBC Two's Newsnight the options included "giving police powers to take fingerprints on fingerprint scanners at airports and seaports".
He added: "The government may also include an attempt to return to 90 days' post-arrest detention.
"If they do that it will, as the home secretary has said, have to be on an evidence base."
Compromise
Metropolitan Police Commissioner Sir Ian Blair has said an extension should be "examined again in the near future", with Chancellor Gordon Brown adding that he "completely" agreed with this analysis.
However, Conservative leader David Cameron accused ministers of peddling the "politics of fear".
'Internment'
The Tories say there should be an extension of the detention limit only if there is "credible evidence" it is needed.
Shami Chakrabarti, director of the civil rights group Liberty, said: "Ninety days is internment - our nightmare but a terrorist recruiter's dream.
"Why have we seen no moves to allow intercept evidence and other tools to bring terror suspects to justice?"
Last week, MI5 chief Dame Eliza Manningham-Buller said the security service knew of 30 terror plots threatening the UK and is keeping 1,600 individuals under surveillance.
Meanwhile ex-Home Secretary David Blunkett has accepted the public was in danger of having "terror fatigue".
He told BBC Radio 4's Today programme on Thursday that raising awareness to the correct level, without creating panic, was "almost impossible".
Mr Blunkett also accepted that the fact the government's intelligence had turned out to be "wrong" about Saddam Hussein's weapons of mass destruction had "undermined confidence in other parts of the counter-terrorism thrust".
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