But in a concession to Labour critics, he is promising a "sunset clause" to the legislation so the controversial scheme will be reviewed by MPs in a year's time.
The Government will press ahead with the 90-day proposal in a Commons vote tomorrow, but has put in place a fall-back position if the Government is defeated.
Former minister Janet Anderson is to table an alternative amendment to the Terrorism Bill which would raise the maximum detention limit from the current 14 days to 60 days.
The moves emerged after intensive negotiations between Labour MPs and Charles Clarke, the Home Secretary. One Home Office source said: "He believed he made headway with backbenchers."
Mr Blair admitted yesterday he faced defeat over the 90-day plan, but told a Downing Street press conference he believed any compromise forced on him by Labour, Tory and Liberal Democrat MPs would be "a compromise with the nation's security" in the wake of the July bombings in London.
Mr Blair's suggestion of a "sunset clause" took Home Office officials by surprise. As they hurriedly began work on the proposal, one source said: "Thanks a lot, Downing Street."
The Government's decision to dig in its heels over 90 days also threatens to infuriate Labour MPs who believed the Government was backing off from the proposal.
Mr Blair also sought last night to bridge a dangerous gulf developing between him and Labour MPs by addressing their weekly meeting. As well as pleading for support on the Terrorism Bill, he tried to reassure them that the reforms he wants to push through on education, health and welfare would help the party win a fourth successive election after he stands down.
"It's only when you do difficult things as a government that the public thinks you have got it in you to govern," he said.
The Prime Minister sought in his press conference to pin the blame for denying the police the powers they wanted on the Tory Opposition. He said when Labour was in opposition, he would have been reduced to "a little puddle of water" had he refused to back toughened anti-terror laws.
But Michael Howard, the Tory leader, recalled Mr Blair had three times voted against measures to combat Irish terrorism between 1994 and 1996. "His comments demonstrate how out of touch with reality he has become," Mr Howard said. He said a 90-day detention limit was one of the most notorious aspects of the apartheid regime in South Africa.
The Tories insisted that they would not move beyond a 28-day limit.
David Winnick, a Labour backbencher, has tabled an amendment to increase the 14-day limit to 28 days.