Shadow Chancellor Oliver Letwin has slammed plans to introduce ID cards, saying there is no evidence they would help fight terrorism.
The speed of introduction of the scheme will take many by surprise
He warned the move could prove costly and affect civil rights, while its benefits were still questionable.
Home Secretary David Blunkett has said proposals for ID cards should be published within a month.
He admitted there had been Cabinet misgivings, but said it would help tackle the terror threat.
But Mr Letwin said the government had not produced "the slightest evidence" that ID cards would make a substantial contribution to the fight against terror.
"I really worry about whether we are embarking on a course of action that will be expensive and complicated and has civil liberties implications and will ultimately be unproductive," he said speaking on BBC Radio Four.
"We really ought to be very careful before we go down that road that there is a practical outcome that is worth a candle," he added.
Mr Letwin conceded that a series of plans to introduce driving licences, passports and cards with fingerprints and other biometric information might not be disastrous in themselves.
But he warned that in 20-30 years' time, British citizens might find their freedoms have been eroded.
Mr' Blunkett aims to create a national database by 2007/2008 containing biometric information, but carrying the new documents would only become compulsory after 80% of the population has got one.
Officials expect to reach this target by 2013.
The Home Office later insisted that ID cards would be a valuable tool in fighting terrorism.
"Hi-tech biometric ID cards will help us with a number of law enforcement issues, including terrorism," a spokesman said.
"Terrorists use false identities to help finance their activities in the UK and abroad.
"False identities are used to launder money, to abuse the immigration system through people trafficking and to facilitate drug smuggling.
"Disrupting their activities is a key priority. ID cards will help us do this."