Home Secretary Charles Clarke has said he is confident MPs will back plans to allow police to hold terror suspects without charge for up to 90 days.
Police chiefs say they need the 90-day detention powers
Ministers are now sticking by their original plan, although they also have a "fall back" position of 60 days.
Mr Clarke said he had made "substantial concessions", including a "sunset clause", meaning the new laws have to be renewed by Parliament after a year.
The Tories and Lib Dems will oppose the plan when MPs vote on Wednesday.
Last week, the government delayed a vote on the new detention powers amid concern about the proposals on Labour's back benches.
But Tony Blair and Mr Clarke decided to press ahead with the 90-day proposal after meeting the Parliamentary Labour Party on Monday evening.
If the vote fails, senior Labour backbencher Janet Anderson will present an alternative proposal which will give MPs the option of voting for detention lasting 60 days.
Mr Blair's official spokesman said on Tuesday: "There is a growing acceptance for 90 days but the government takes absolutely nothing for granted."
Mr Clarke told BBC News significant numbers of MPs had been convinced by the police case for longer detention.
Mr Blair says anything less than the 90-day plan would be "second best" for the UK's security.
He won support from Chancellor Gordon Brown, who urged Tory leadership contenders David Cameron and David Davis to put the advice from the security services first - "before any political opportunism".
The Lib Dems want the maximum period in custody without charge to remain at 14 days, while the Conservatives are arguing for 28 days.
Conservative shadow attorney general Dominic Grieve dismissed the "sunset clause" concession, saying it "made a mockery of the assurances that Charles Clarke gave earlier".
"It suggests a major split between the home secretary and the prime minister and it shows that the government has behaved with a degree of deceit in dealing with parliament," added Mr Grieve.
But some Conservative MPs, including former minister Ann Widdecombe, say they will vote for the government's 90-day proposal.
Lib Dem leader Charles Kennedy predicted ministers would not get the 90-day plan through Parliament.
"It is reprehensible and regrettable that the prime minister is just throwing consensus out of the window - or indeed any genuine efforts to achieve consensus," said Mr Kennedy.
The Metropolitan Police have released a seven-page memorandum on the issue.
But John Denham, the Labour chairman of the Commons home affairs committee, said a more rigorous analysis was needed.
Human rights lawyer Michael Mansfield QC said the police already had enough powers and the problem before the 7 July attacks in London was an "intelligence vacuum".
He suggested the new powers would lead to miscarriages of justice.
Labour backbencher David Winnick is reintroducing an amendment calling for a maximum detention time of 28-days.
Mr Winnick withdrew that amendment last week after Mr Clarke offered all-party talks, when it became clear the proposal faced a Commons defeat.