Michael Howard has signed Labour's petition on keeping the NHS free.

The Conservative leader said he was signing it because he was "totally opposed" to the principle of charges being introduced for operations.

Labour claims that the Tories would introduce charging into the NHS.

Liam Fox, the Conservative Party co-chairman, later confirmed that under a Conservative government the NHS would pay for half the cost of a private operation even if the patient involved had private health insurance.

Alan Milburn, Labour's general election co-ordinator, said the policy would involve taking £1bn out of the NHS and paying it into the pockets of those wealthy enough to afford private medical insurance.

Meanwhile, the Tories claimed Labour was lying to the public about tax rises for extra health spending after its own manifesto said it decided to increase National Insurance in 1998 - four years earlier than the policy was made public by ministers.

Labour dismissed the statement as a "typographical error".