About 24,000 people in the UK are carrying the agent that can cause the deadly brain condition Creutzfeldt Jakob disease (CJD), twice the number scientists previously estimated.
The latest figure is based on a study of 30,000 appendixes removed in operations, which were tested for the presence of the prion, or misfolded protein, that causes the condition, linked with eating infected meat.
More than a decade ago ministers assured the public that beef was safe to eat, and then had to eat their words when, in March 1996, it was announced that a new disease, variant CJD, had been discovered in humans.
It had come from eating meat from cows infected with bovine spongiform encephalopathy (BSE), a similar disease. BSE became known as mad cow disease, and in humans CJD is characterised by rapidly progressive dementia and death.
However, only a small proportion of people who carried the prion developed the clinical disease. There have been 173 cases of variant CJD in the UK since it was identified in 1996.
The number of carriers of the prion is significant because there is a theoretical risk they could spread the disease through blood transfusions or surgical instruments. Tough measures are in place to minimise these risks.
The Health Protection Agency, which published the new figures, said one in 2000 of the adult population of Britain were carriers of the condition.