Plans to introduce identity cards risk ending up as a "monument to the failure of big government", Conservative leader David Cameron has warned.
Mr Cameron questioned the value of ID cards
His comments, during prime minister's questions, follow a report which estimates they will cost £14.5bn.
Tony Blair said the entire government "absolutely" backed ID cards and fears over civil liberties were "misplaced".
Plans for ID cards were defeated in the Lords on Monday, but ministers say they will press ahead with them.
The prime minister said they were needed to fight identity fraud and illegal immigration.
Mr Cameron, pointing to reports that Gordon Brown was opposed to ID cards, asked if the scheme would go ahead when the chancellor succeeded Mr Blair as prime minister.
Mr Blair replied: "I certainly can give a guarantee that the government as a whole is absolutely behind identity cards ... "
On Monday, peers voted to block the scheme until its full costs were known.
They also voted for more security provisions, and for more controls on who can access the data.
Mr Cameron cited a report by the London School of Economics, which claims the scheme would cost between £10bn and £19bn over 10 years if the government followed its original plans.
Mr Blair said that report was drawn up by someone who was a campaigner against ID cards on civil liberty grounds.
The Home Office estimates the scheme will cost about £584m to run each year, with each combined biometric passport and identity card costing £93.
'Good use of money?'
However, it has not given full cost estimates for setting up the scheme and says its overall costs will depend on how government departments choose to use the card scheme.
Mr Blair said most of the costs would be covered by those already incurred by the introduction biometric passports.
Mr Cameron asked: "With rising deficits in the NHS, huge costs of pension reform and tighter pressures on public spending, how can you claim that spending at least £600m a year on your ID cards scheme is a good use of public money?"
Mr Blair replied: "Because if we introduce an ID cards scheme and reduce identity fraud that makes a major difference to the costs of government, to the costs of doing business.
"In today's world if we want to tackle illegal migration, crime and identity fraud, then using the new biometric technology to have ID cards is an important part of doing so."
The government is likely to try to overturn the Lords defeats when the ID cards bill returns to the Commons.