The Nuffield Orthopaedic Centre in Oxford could be stripped of its top NHS rating due to a rise in cancelled operations.

Managers at the Headington hospital have warned that their three-star rating could be jeopardised by a sudden increase in patients whose surgery is cancelled at the last minute.

The NOC was awarded the highest achievable mark in September, when all hospitals were graded from zero to three.

One of the key targets used in the NHS ratings was cancelled operations, a problem which has re-emerged at the NOC.

In July to September last year, 17 people were told they could not have surgery. But during the same months this year, 37 people were turned away.

In a report for the NOC executive board, managers warned that "under-achievement" in cancelled operations meant the trust would only be given two stars next year.

NOC nursing and operations director Jan Fowler admitted the trust had a problem, but said all affected patients were re-admitted within one month.

She said: "In the last two quarters we've had higher than normal cancellations.

"The main reason is that we have a number of medically unfit patients. Although they're screened beforehand, on the day a cold or something prevents their surgery from taking place."

The NOC is now improving pre-admission screening. Managers are also reforming theatre lists so all operations are fitted in during a shift.

It is thought ratings for 2003 will be based on performance next March, and managers are using indicators from this year's ratings to monitor progress.