The Conservatives say they would scrap ID cards
The Tories have written to five firms bidding to supply ID cards warning them not to sign any long-term contracts.
In the letter, shadow home secretary Chris Grayling says one of his party's first acts, if it wins the next general election, would be to scrap the scheme.
He said he was urging the firms against large investments that may be wasted.
The government says ID cards, being trialled in Manchester from this autumn, will combat fraud, terrorism and organised crime.
They want a nationwide roll-out of the scheme by 2012 but with a general election due within a year, the Conservatives say they intend to scrap it.
Mr Grayling's predecessor as shadow home secretary, David Davis, issued a similar warning to firms in February 2007 and gave Cabinet Secretary Sir Gus O'Donnell formal notice of the party's intention not to continue with the scheme.
The Conservatives and Liberal Democrats oppose the scheme, estimated at costing about £5bn, and some Labour MPs have expressed doubts.
ID CARD TIMETABLE
2009: Workers at Manchester and London City airport
Autumn 2009: Manchester pilot
2010: Students opening bank accounts offered ID cards
2011/12: All UK passport applicants
2015: 90% foreign nationals covered
2017: Full roll-out?
Mr Grayling told BBC Radio 4's Today programme he was concerned about "a number of signals" recently suggesting "quite big penalty costs" were being built into contracts which will leave a "substantial bill" for the taxpayer.
"I want companies to be cautious and recognise that if they invest large amounts of money preparing for this business, it may not happen," he said.
"There's a danger the government will build more poisoned pills into the contracts that will simply make it more difficult to scrap."
Asked whether the Tories were trying to paralyse government plans, he said: "I would be delighted if this slows down progress with the ID project because I think it's the wrong thing to do."
Later this year, airside workers at London City and Manchester airports will be issued with ID cards.
And, from the autumn, people in Manchester will be able to voluntarily sign up for a card as part of a pilot project.
It is the beginning of the main phase of the scheme which ministers say will result in cards being available nationwide by 2012.
Within the next three years, the Identity and Passport Service plans to issue "significant volumes" of ID cards alongside British passports - but people will be able to opt out of having a card if they do not want one.
Earlier retired law lord Lord Steyn accused the Home Office of introducing the cards in stages as a way of "conditioning" and "softening up" public opinion.
He added: "The Home Office now proudly asserts that comprehensive surveillance has become routine. If that is true, the resemblance to the world of Kafka is no longer so very distant."
The government believes that the public support the scheme - former home secretary Jacqui Smith said she was regularly approached by people who said they did not want to wait several years to register for an ID card.
It has been reported that Alan Johnson, who replaced Jacqui Smith as home secretary in the recent cabinet reshuffle, might be considering a U-turn on ID cards, after ordering a review of the scheme.
But in a statement Mr Johnson said: "In my very first interview as home secretary I made clear that identity cards was a manifesto commitment and that legislation governing their introduction was passed in 2006.
"We remain on progress to bring in what we believe has widespread public support."