The Shadow home secretary David Davies has asked the government to agree get-out clauses with companies it contracts to build its ID card systems and databases so that a Conservative government can ditch the scheme.

In a letter to Sir Gus O'Donnell, cabinet secretary, Davies gave "formal notice" that the Conservatives would scrap the ID card and drew on the gentleman's agreement between successive governments than one won't tie the hands of its successor.

"As a matter of financial prudence, it is incumbent upon you to ensure that public money is not wasted, and contractual obligations are not incurred, investing in a scheme with such a high risk of not being implemented," he wrote.

"I would be interested to know what provision, if any has, been made in the relevant contractual arrangements to protect the Government - and public funds - against the costs that would be incurred as a result of early cancellation of the scheme."

The Identity and Passport Office said the Labour Party, rather than the government, was preparing an answer.

The Cabinet Office said they only got the letter this afternoon so they hadn't had time to prepare a response.