The patients all underwent surgery with instruments that had been used on a patient being tested for CJD, the human form of “mad cow” disease.

The instruments should have been quarantined after that operation, but were used in 29 further cases in what the Health Department described last night as an “appalling incident”.

The discovery was made after a patient underwent a brain operation at Middlesbrough General Hospital on July 19. Sporadic CJD was diagnosed on August 8.

Doctors immediately informed the patient, who has not been identified, and the equipment was withdrawn. But by then it had been used on 29 other people.

Under government guidelines, the instruments should have been quarantined as soon as the risk of vCJD was evident and not once it had been confirmed, at which point such instruments are withdrawn, the Health Department said.

Since the CJD case was identified, the South Tees Hospitals NHS Trust has been working closely with the CJD surveillance unit to assess the risk to the 29 patients, who were contacted last night. The hospital emphasised that the risk was “extremely low”.

A spokeswoman for the trust said: “We were advised not to contact the patients concerned until further guidance was given, which was issued this afternoon but are now in the process of meeting every individual to fully explain their unfortunate circumstances.

“No operations had to be postponed as a result because replacement new equipment delivered to the trust was immediately available.

“We appreciate the distress and concern this news may cause to these patients, their families and the public at large, and we are in the process of setting up a helpline.”

Sporadic CJD occurs in people who have no family history of the disease, and it strikes without any apparent reason. It accounts for about 85 per cent of all cases of the illness, but should not be confused with variant CJD, which is the human form of BSE that has been linked to contaminated meat.

The Government admitted earlier this month that millions of Britons could have been exposed to the risk of CJD from low-cost burgers and minced beef and there have been more than 110 cases of vCJD in Britain since 1996. At least 100 people have died of the disease, which has an incubation period of up to 40 years.

The Government also gave warning last year that thousands of people who had had surgery were at risk from the disease since the prion agents that cause CJD are virtually impossible to eradicate from surgical instruments.

But a ban on reusable surgical tools in tonsil operations was lifted after concerns were raised about disposable instruments.