Despite the terrorist attacks in London, doubts are growing inside the Government about the cost and practicalities of the controversial proposal which is now being pushed through Parliament.
The Chancellor will resist any attempt by supporters of ID cards to use the London attacks as an excuse to speed up their introduction. Significantly, Charles Clarke, the Home Secretary, admitted yesterday that cards would not have prevented Thursday's bombings.
The Home Secretary said: "I doubt they would have made a difference. I don't argue that ID cards would prevent any particular act. The question on ID cards or any other security measure is: does the particular measure help or hinder? I actually think ID cards would help rather than hinder.
"But if you ask me would ID cards or indeed any other measure have stopped yesterday, I can't identify any measure that would have stopped it like that."
Last month, the Home Office rubbished a London School of Economics study which said the scheme was "technically precarious" and put the cost at between £10.6bn and £19.2bn, which could almost double the Government's £93 estimate of the cost of a card.
Mr Brown does not oppose ID cards in principle but is said to be worried that the technology might not be proven and that the costs could rocket. Treasury officials will now take a close look at the figures in the light of the 300-page LSE report.
Allies say Mr Brown, the overwhelming front-runner to succeed Mr Blair, would want to make some symbolic breaks with the Blair era if he becomes Prime Minister. They believe that scrapping ID cards is likely to be one of them.
Charlie Whelan, Mr Brown's former press secretary, said the ID card scheme would get an "early bath" if Mr Blair stood down soon. Writing in the New Statesman magazine, he asked: "Does anyone seriously believe that Brown will back this bonkers idea?"
Mr Blair supports the project, which was approved by the Cabinet when David Blunkett, its leading advocate, was Home Secretary. But some Blairites are having doubts. One said: "I am not convinced the technology will work. We can't afford another computer fiasco."
The Prime Minister and Chancellor will watch public opinion closely in the wake of Thursday's bombings. Recent polls have suggested that support for ID cards is slipping amid fears about how much they would cost.
Originally, ministers cited terrorism as the primary reason for backing the scheme but they have recently argued that ID cards would also combat identity fraud and ensure that British passports can be used in countries such as the United States.
Last month, Mr Blair said he did not come into politics to bring in ID cards and hinted that he would abandon the scheme if the practical difficulties became too great.
One option would be to press ahead with the legislation to avoid a humiliating climbdown but not to implement it on the grounds that there were too many unresolved problems - a course that might appeal to Mr Brown.
David Davis, the shadow Home Secretary, said the London bombings had not changed the Tories' opposition to ID cards. He said: "The Home Secretary admitted himself that ID cards would not have prevented this and may not even have helped in the investigation afterwards so it doesn't change the argument." He added: "An important part of our way of life is maintenance of our civil freedoms."