Barnet residents who visited the Royal National Orthopaedic Hospital between 1995 and 2002 have been warned they might have been infected with hepatitis or HIV during investigative surgery.

The warning affects people who underwent probes on their shoulders at the Stanmore hospital's Bolsover Street outpatient clinic in central London.

The hospital stressed that the risk is very small and theoretical as none of its patients was known to be carrying either HIV or hepatitis. But it has advised that worried patients should come forward for blood tests.

All 551 patients concerned have been written to, and almost 100 of them have already contacted the hospital's hotline. The trust, however, said it had delayed informing patients for two years while it consulted with the Health Protection Agency (HPA) on the risk of infection.

It had been discovered that some of the probes were used on more than one patient, and although they were cleaned with alcohol, it is thought this may not have been enough to provide protection. An orthopaedic surgeon has been referred to the General Medical Council.

The chief executive of the RNOH trust, Andrew Woodhead, said the HPA told them the risk of infection was very low.

He added: "But our board was firmly of the view that our patients had the right to know this procedure was not carried out correctly, so we have written to every patient for whom we have records.

"For those who are concerned we are offering a blood test to detect viruses and have set up a number of clinic sessions to undertake." A helpline has been set up at NHS Direct on 020 8867 1426. It will operate from 8am to 8pm seven days a week.

The investigations were undertaken on NHS and private patients as part of a research programme. A probe was inserted into a tube in the shoulder and the patient asked to move the joint while the pressure was measured.

In most cases there was no likelihood of the probe coming into contact with the patient's blood, but, if it did, and a previous patient had a blood-borne disease, there was a risk of infection.

Mr Woodhead said: "We believe we let our patients down by not following the usual high standards that we expect.

"I am pleased that a member of our staff alerted us and I believe the trust has reacted properly and done all it can."