Police officers and counter terrorism experts on Monday sounded a warning to David Blunkett, after the home secretary said the government was considering introducing compulsory identity cards.

Britain - along with the US, Canada, Australia and New Zealand - is unusual among developed nations in not having some form of identity card. At least 11 of the 15 European Union countries have them, ranging from a voluntary card in France that almost everyone carries, to compulsory cards that must be carried at all times in Germany, Spain, Greece and Belgium.

However, when the last Conservative government trailed the idea in a green paper five years ago, the Association of Chief Police Officers favoured a voluntary card, fearing that giving the police the right to demand a compulsory one would damage relations with the public.

The association on Monday acknowledged that the situation had changed, but said there remained "a deep seated resistance to the notion that an ordinary citizen should require some form of identification which they would have to carry around the streets".

It was, however, right for the government "to look at some of these fundamental issues again and examine them against the nature of the threat we now face".

A modern, national identity card - something more than an easily forged plastic photo card - would be expensive. Bill Waller, marketing manager for Gemplus, a manufacturer of microchip-embedded smart cards, said a single smart card could provide access to a range of government and private services from driving licences and social security benefits to banking.

The government is examining the potential of that for a range of services other than a national identity card. Digitised photographs and fingerprints, or iris recognition systems, would make them difficult to forge, he said.

However, while an individual card might cost only £4 to £5, installing a national identity card system with all the readers and systems back-up would cost "several billions".

Professor Paul Wilkinson, director of the centre for the study of terrorism and political violence at the University of St Andrews, said: "I would be worried if people thought that this was the answer to problems of international terrorism."

Machine-readable, compulsory identity cards in Spain had not stopped Eta, the Basque terrorist group. Sophisticated cards might "in the long run" make it easier to track the movement and financial transactions of someone attempting to infiltrate a country, once they were identified, "but it is not a panacea".

"People coming from abroad would not be expected to have a British identity card and people could live here apparently innocently for a period of time as students or visitors, but be the sleepers waiting to commit the kind of outrage we saw in New York.

"It would be foolish to claim it was a guarantee against terrorism. All it would do is assist the police and the criminal justice system, in the longer term, to trace citizens of our own country who, for some reason, were engaged in criminal or terrorist activity."

Without a secure global identity card, "then it is not going to be the highly effective device that people hope it might be."