Technology behind the government's controversial identity card scheme must be "almost foolproof", the UK's most senior police officer has warned.
Ministers say ID cards could protect people from identity fraud
The cards could tackle terror only if biometric indicators like irises and fingerprints were recognised almost perfectly, Sir Ian Blair said.
The Met commissioner, addressing the London Assembly, defended his pre-election support of ID cards.
They could stop the "day by day" increase in identity theft, he added.
He did not want to see Met officers "demanding to see people's papers" at random in the street.
But it was "a danger to the state" that the government did not know who some people were, he told Wednesday's meeting.
"ID cards can only be the answer if the recognition of them is almost perfect," he said.
"Identity cards are only going to work if we have a biometric answer - that may be iris recognition but it is unlikely to be facial recognition because that changes because of diet and beards and everything else."
The technology had to be "as close to foolproof as possible", Sir Ian said.
"There are a whole pile of logistics that have to be got right."
Asked whether he regretted publicly supporting ID cards in the run-up to the May election he said he was "not in the slightest bit repentant".
He spoke out on the issue after the conviction of ricin plotter Kamel Bourgass, whose real identity is not known by authorities.
"I am absolutely clear that in the aftermath of the Bourgass trial with commentators saying this proved there was not a threat to the UK, it was the job of the commissioner to say this was a real and present threat," he said.
He told the meeting nothing had changed to alter the level of danger from terrorism in the UK and said people should remain "alert not alarmed".
"We must be very concerned about complacency in governance and in public terms because if we drop our guard trouble is out there," Sir Ian said.
Meanwhile, the commissioner repeated his call for late bars and nightclubs to help pay for late night policing in city centres to tackle alcohol-fuelled disorder.
"City centres are full of places which are sometimes described as drinking factories," he said.
"If you want to drink until three in the morning you are a special interest group and you have got to pay for it."