The Tories have claimed that half the adult population will have to sign up to ID cards before the scheme will achieve the government’s objective to pay for itself.

The estimate came from Conservative shadow home secretary Chris Grayling as the latest Home Office figures showed the cost of the National Identity Register and card scheme over a rolling five-year period has fallen by a small amount, down to £4,575m from £4,580m six months ago.

The Home Office is obliged by legislation to produce five-year cost estimates every six months. They exclude the cost to individuals of providing data to verify identity, estimated to run at £30 a head for fingerprints and a biometric-compliant photograph.

Grayling denounced as "deluded" a claim from home secretary Alan Johnson that scrapping the scheme now would save nothing because it was expected to become self-financing.

He said: "If Alan Johnson seriously thinks that nearly half the adult population is going to voluntarily pay for ID cards out of their own pocket then he is completely deluded. It is time the government realised this whole scheme is a white elephant and it should be scrapped immediately.”

ID cards will become available in Greater Manchester later this year but the government has ruled out making them compulsory for at least five years. So far, only 10,000 people have registered an interest in purchasing a card.

The Tories are committed to scrapping the scheme if elected next year.