Patients are being suspended from hospital waiting lists while they go on holiday.

One London hospital has sent patients a form asking them about their holiday dates. When they reply, they are told their holiday period will be added to their waiting time. NHS figures do not show that patients who have gone on holiday have had target waiting times "frozen".

Chris Grayling, the Tory health spokesman, said forcing seriously ill people to choose between a holiday and early treatment was "unacceptable". He said it was "symptomatic of the focus on meeting ministerial targets rather than treating patients".

"From a patient's point of view, to send them a letter with an innocuous inquiry about a holiday and then tell them it will affect their treatment is totally unacceptable," he said. "It is another way for the health service to make the figures look better. It is happening all over the place."

Mr Grayling said he had been contacted by a woman awaiting admission to the National Hospital for Neurology and Neurosurgery in central London. She was sent a letter by hospital administrators asking whether she was going on holiday during the time set aside for her operation. The letter threatened to remove her name from the waiting list if she did not reply within a fortnight.

Once the woman replied, she was told her waiting time had been "frozen" and she had been suspended from the waiting list for the period of her vacation.

The Tories believe hospitals are exploiting an obscure NHS rule which allows holidays to be considered as a "suspended" period in official waiting times. Official NHS guidance, which predates the 1997 election, allows hospitals to suspend patients if they say they are going on holiday.

University College London Hospitals NHS Trust, which runs the National Hospital for Neurology and Neurosurgery defended its policy, but said its "use of language" may have been a mistake.

"We do ask patients if they are going on holiday. This is purely to ensure that we do not schedule their admission dates during that period," said a spokesman. "Because they are suspended for this period in accordance with guidance from the NHS information authority it is added to their waiting times. Waiting times at University College London Hospitals NHS trust are less than the NHS maximum of 12 months."

John Hutton, the Health minister, said that "the time the patient is away on holiday counts as a period of suspension". He told Mr Grayling: "The waiting time is calculated from the decision to admit date to admission date or end of reporting period and any periods of suspension deducted from the total length of wait."