The minister in charge of the ID cards scheme has played down fears from an ex-MI5 chief that the cards may not help fight terrorism.
Stella Rimington said ID cards were unlikely to make Britain safer
Dame Stella Rimington said most documents could be forged and this would render ID cards "useless".
But minister Andy Burnham said the idea of the scheme was that the cards would be "almost impossible to forge".
The Conservatives said Dame Stella's words showed the case for the scheme was "bogus".
The Lib Dems say the controversial plans should be abandoned.
'Waste of money'
The government has argued the cards scheme will help prevent terrorism, organised crime, illegal immigration and fraud.
Dame Stella retired as head of MI5 in 1996 but gave her personal views on the cards at the Association of Colleges annual conference in Birmingham.
She said: "ID cards have possibly some purpose.
"But I don't think that anybody in the intelligence services, particularly in my former service, would be pressing for ID cards.
"My angle on ID cards is that they may be of some use but only if they can be made unforgeable - and all our other documentation is quite easy to forge.
"If we have ID cards at vast expense and people can go into a back room and forge them they are going to be absolutely useless.
"ID cards may be helpful in all kinds of things but I don't think they are necessarily going to make us any safer."
'Major step forward'
In an interview for the BBC News website, Mr Burnham said he did not accept "for a second" the national identity database linked to the card would be "of no value to the security services".
He said the premise of the system was "that this will make it impossible - impossible is a big claim - it will make it almost impossible to forge an identity".
The current identity system, which included passports being forged, was no longer up to the job, he argued.
He said biometric technology, where fingerprints, iris scans and facial images are stored on a database linked to the card, was potentially a "major step forward".
"The crucial strength that it brings is that an identity... can only be registered once, whereas at present people can register many times under different names."
Mr Burnham said technical trials of the system had not yet taken place so he could not say the biometrics were unforgeable.
But he pointed to the success of American biometric immigration controls.
The minister said he did not know whether the intelligence services would be pressing for the cards, nor did he want to speak for the current MI5 chief.
He highlighted how former Metropolitan Police Commissioner Lord Stevens had presented a "compelling" argument for the plans in a recent House of Lords debate.
Conservative shadow home secretary David Davis seized on Dame Stella's comments.
"Stella Rimington would know better than any government minister the security value of ID cards," he said.
"What she has demonstrated is that they will be a spectacular and probably counterproductive waste of money - an unnecessary incursion on people's privacy."
Lib Dem home affairs spokesman Mark Oaten said Tony Blair had urged MPs last week to listen to the experts on terrorism laws.
"When someone of Stella Rimington's experience expresses these doubts, the government should listen and abandon this scheme before wasting any more taxpayers money," he said.
"Tony Blair is always saying we should listen to the experts, this time he should be listening."
Former government crime advisor Lord Mackenzie disagreed with Dame Stella's assessment.
He told BBC News: "Let's look at the Soham murders. If Ian Huntley had had an identity card, would he have got the job at Soham school which allowed him to commit the murders? I think not."
But Shami Chakrabarti, director of civil liberties pressure group Liberty, described the former security chief's comments as "another nail in the coffin of the massive identity card folly".