The Conservative Party has written to the five companies in the running for the ID cards contract, warning them not to sign any large contracts.
The party says one of its first actions if elected to government would be to scrap the scheme and shadow home secretary Chris Grayling told the companies not to make any large investments that may be wasted.
ID cards are being introduced this year, with airport workers in Manchester being the first to trial them. The scheme is controversial, attracting criticism from privacy campaigners and prompting pilots to refuse to take part in trials. The government says the cards will help fight terrorism and other crime.
The cards will be rolled out nationwide by 2012 according to the government's timetable, but a general election is due before then and could change the plan.
Mr Grayling told BBC Radio 4's Today programme he was concerned about recent suggestions that "quite big penalty costs" were being built into contracts. This could leave a "substantial bill" for the taxpayer if the scheme is subsequently scrapped.
He said, "I want companies to be cautious and recognise that if they invest large amounts of money preparing for this business, it may not happen."