After a report in The Observer newspaper, a government spokeswoman said that Blair would accept this week that he could not push the tough new anti-terror powers through parliament.
"The prime minister acknowledges there will have to be negotiations and/or compromise; but as far as he is concerned 90 days continues to be right," the spokeswoman said.
Last week, Home Secretary Charles Clarke was forced to promise fresh talks after it became clear that opposition members of parliament - and some backbenchers from Blair's Labour party - would vote down the measure.
Clarke is to meet his counterparts from the main opposition Conservatives and smaller opposition Liberal Democrats on Monday.
Blair also plans to address rebel legislators himself on the importance of the issue while British police forces were being urged to put pressure on lawmakers to back the greater detention period, several weekend newspapers reported.
But the Independent on Sunday quoted Mark Oaten, the Liberal Democrats' home affairs spokesman, as saying: "They are not going to get this bill through unless the Home Secretary moves significantly on 90 days.
The 7 July blasts killed 56 people
including the bombers
"I can guarantee this Bill will be defeated without that. There has to be give and take on all sides."
Despite the apparent climbdown, Blair reiterated his belief in the 90-day proposal, which was recommended by police after the deadly 7 July attacks in London that killed 56 people, including four bombers.
Currently, terror suspects can be held for up to 14 days before they must be released or charged.
"I still think there is a woeful complacency about a lot of the public debate about this," Blair said in an interview in The Sunday Telegraph.
"The police told me, and the security services back them up, that they may have stopped two further attempts since 7 July.
"I find it really odd that we're having to make the case that this is an issue, when virtually every week, somewhere in the world, terrorists loosely linked with the same movement are killing scores of people."
He was backed by Andy Hayman, head of anti-terrorism operations at London's Metropolitan Police.
"All of the new elements mean that in the most complicated cases, there must be the opportunity to extend detention before we make the decision to charge or release a suspect," he told the News of the World.
Charles Clarke was forced to
promise fresh talks
Blair acknowledged it would be a setback for the government if members of parliament voted against the measure when the Terrorism Bill returns to the lower house of parliament for debate this week.
"I will feel a sense of a defeat not so much for me, as it were - although obviously that's true - but for the security of the country," he told The Sunday Telegraph.
Opponents have sought to cast doubt on Blair's authority following the resignation on Wednesday of one of his key allies, the Work and Pensions Secretary David Blunkett, over his business dealings.
Blair told The Sunday Telegraph he was frustrated that claims about the erosion of his authority were diverting attention away from the issue of national security.