The cost of introducing a national identity card will be impossible to gauge until the government is clear on what it will be used for, say experts.

Home Secretary David Blunkett wants plans for paving legislation on ID cards to be introduced in the Queen's Speech in November. He wants the scheme to be compulsory and for people without cards to be ineligible for public services such as healthcare.

Since the concept of an 'entitlement' card was first mooted last year, plans have changed from a card authorising access to social security benefits to an obligatory identity for everyone over the age of 16.

But until a clear specification is produced it is impossible for the government to do a cost/benefit analysis, says Peter Sommer, security expert and research fellow at the London School of Economics.

Roll out costs will be dependent on the infrastructure for checking the cards, says Sommer.

'These are huge variations in terms of cost so until the government is clear on what they are trying to achieve and what the technical infrastructure will need to be, we don't know what the costs will be,' he said.

'It is highly unlikely a cost/benefit analysis has been worked out because going back through the consultation papers, the agenda keeps changing. You can only work out a specification if there is a clear agenda of what the government wants to do.'

The design will also need to take into account people visiting the UK without cards.

'At any time large numbers of people are legitimately in this country who are not part of the scheme - tourists, business people, foreign students - how will the system cope with that?' said Sommer.

Mike Davis, analyst at Butler, says the logistics of implementation are 'horrendous'.

'The sheer physicality of issuing 30 to 40 million cards is phenomenal.

'If you live in the Shetland Islands you can still apply for a passport. But to have a retinal scan done you will have to go to the mainland. The cost logistics are horrendous,' he said.