The government's expectation that most Britons will have a biometric identity card within nine years seems likely to be wide of the mark unless they are made compulsory.

The announcement by Home Secretary Jacqui Smith that the scheme would be "rolled out" beginning with foreign nationals, followed by airport workers, was designed to reinforce the idea that this is a measure which will safeguard national security. The third tranche of ID cards is to be aimed at young people, on a nominally voluntary basis, but with a presumption that by 2010 they will be the preferred form of identity for opening new bank accounts. The thinking is that, by then, the general public will be persuaded that the inclusion of biometric data on their passports will be an advantage. All this assumes, however, that they buy the argument that a plastic card containing fingerprints as well as a photograph, name, address and date of birth will prevent fraud and terrorism, and make life easier.

Following the loss of computer discs with the details of 25 million families who receive child benefit, however, there is general distrust of the government's ability to protect information stored in data banks. Ms Smith sought to provide reassurance by emphasising that personal details and biometric information will be stored on separate databases. That, however, will require verification by cross-referencing, which opens up the system to the possibility of human error. Already there are thousands of examples of cross-checks on criminal record databases leading to people being wrongly excluded from jobs.

The cost of the scheme is another negative factor. Estimates vary from £5.6bn over 10 years, according to the government, to well over twice that amount. Either way, it is a hugely expensive project, especially at a time of tightening public spending, and can only be justified if it will deliver benefits that cannot be achieved in any other way. It is difficult to see how that is the case. An identity card may be a simple way for foreign nationals to prove that they are legally entitled to work in the UK, but they already require passports to be here. All airside workers at airports also already require identification and air crew are subjected to regular security monitoring. The deepest concern, however, is over the integrity of the biometric information. In theory, fingerprints are unique, but there are enough examples from the criminal justice system of dubious matches to suggest that biometric information is not 100% foolproof. It is certainly not tamper-proof and the real danger of a system that relies on ID cards is that false ones become a bigger security risk than none.

In discouraging public services and businesses from asking for ID cards, the Scottish Government (backed by other parties) is reflecting widespread unease that once they are introduced on a voluntary basis, they will be so routinely asked for that they become de facto compulsory. It is an important stand against their introduction by stealth.