Half the UK population will need to sign up for ID cards for the scheme to become self-financing, according to the Conservative Party.
The government has consistently said that the costs of its National Identity Scheme will be met by application fees. However, on Monday the Conservatives said they had collated available government figures to calculate that ID cards will cost £835m in total to implement. Taking the government estimate of £30 per card, 28 million people would need to obtain one to meet that price tag, the Conservatives said.
"If [home secretary] 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," shadow home secretary Chris Grayling said in a statement on Monday. "It is time the government realised this whole scheme is a white elephant, and it should be scrapped immediately."
The latest government cost estimates, published in May 2009, put the total cost of the scheme at £4.9bn for UK nationals.
However, government estimates for the costs of the National Identity Scheme have long been disputed. In 2005, the London School of Economics said costs could be as high as £30bn.
The Tories said on Monday that Home Office figures showed declining support for the National Identity Service. However, the Home Office remains confident ID cards will prove popular.
"Latest figures show that support for an ID card remains at nearly 60 percent, and so it is not unreasonable to expect 28 million people will choose to get a card in the next 10 years," a Home Office spokesperson commented. "Benefits of the credit-card sized document include travel within the EU and a national proof-of-age card."
A £30 ID card will be a more affordable identity document than a standard passport, which costs £77.50, the spokesperson added.
Government statistics about the popularity of the scheme have also been queried. In June, the government said 59 percent of the population supported it. But according to an October survey for campaign group No2ID, 60 percent oppose ID cards, with two-thirds opposing the system of centralised databases that will lie behind the scheme.