Frozen sperm belonging to 28 cancer patients, whose treatment may have left them sterile, has defrosted, a hospital admitted yesterday.

The incident, at Southmead Hospital in Bristol, has been blamed on a faulty freezer storage tank - one of two on the site used to store the sperm of those undergoing treatment damaging to their fertility. The unit contained sperm samples given by patients over the past 12 months. It is unknown how many of the men have lost the chance to become fathers, but an initial assessment found the sperm samples were "damaged beyond use".

Dr Tim Lewis, clinical support director for the North Bristol NHS Trust, said an internal inquiry into the incident had been launched.

"Unfortunately the initial assessment suggests that the sperm stored for 28 men have been lost or irreparably damaged by the accidental failure," Dr Lewis said.

Yesterday, the men in question were said to be furious at the hospital's handling of the situation. The relevant consultant is away on holiday, and many patients were unsure where to turn for clarification of the situation or support. The patients were told in a letter that if they wanted to discuss their concerns, a consultant would be available on either 14 or 21 August.

One of the men, who did not want to be named, said: "It's heartbreaking. We are already trying to cope with cancer and now this. They are dealing with such a precious cargo, surely it should have been protected by some back-up system or alarm?

"All we've had is a letter saying sorry. They're not even offering us counselling."

Dr Lewis, however, claimed that the hospital would provide as much support as possible for the patients. "We appreciate that this is upsetting and distressing news for the men concerned," he said. "They have already had to deal with difficult and stressful treatments for their cancers. They have either been offered dates to be seen in the clinic by one of the consultant team or to discuss the implications by telephone." The men would also be offered "additional fertility testing and counselling".

The Human Fertilisation and Embryology Authority said it was treating the incident with "the utmost seriousness". An inspection team investigating the incident would be sending recommendations to all clinics to prevent future losses.

Southmead Hospital's second freezer bank, which contained sperm given by more than 400 men before July 2002, was not affected.