The Conservatives are launching a print and internet campaign against the introduction of identity cards.
About £58m has been spent so far on ID cards
It comes after shadow home secretary David Davis wrote to Cabinet Secretary Sir Gus O'Donnell to warn him a Tory government would scrap the scheme.
In the letter, he asked what measures were in place to ensure that early cancellation does not hit taxpayers.
The Tories say cards will damage civil liberties, but the government insists they will improve security.
The letter from Mr Davis, sent on Monday, said the Cabinet secretary was "formally on notice" of the party's intentions.
It added: "You will be aware that there is a long-standing convention that one parliament may not bind a subsequent parliament.
"As you will also be aware, the Conservative Party has stated publicly that it is our intention to cancel the ID cards project immediately on our being elected to government."
Mr Davis also warned the firms likely to bid to run the ID cards project about the party's intentions.
Around £58m has been spent so far on the scheme.
Ministers say it will cost £5.4bn over 10 years, but the Tories estimate the figure will be nearer £20bn.
Critics say the cards, due in 2009 and compulsory for passport applicants from 2010, will breach privacy. But ministers say they will aid security.
Home Secretary John Reid accused Mr Davis and Tory leader David Cameron of trying to "talk tough while acting soft".
He added: "They will the ends whilst constantly opposing the means to protect the people of this country."
Cards will contain basic identification information including the name, address, gender, date of birth and photo of the holder.
Last December, the government abandoned plans for a giant new computer system to run the scheme.
Instead of a single multi-billion pound system, information will be held on three existing, separate databases.
The government says the cards will help tackle illegal immigration, identity fraud, organised crime and terrorism.