About 24,000 people in the UK are carrying the agent that can cause the deadly brain condition Creutzfeldt Jakob disease (CJD), linked with eating infected meat – twice the number previously estimated by scientists.

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.

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 caused by the now discontinued practice of feeding ground-up animal carcases to cows as part of their diet. 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 first 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 which are not properly sterilised between operations. Tough measures are in place to minimise these risks.

The Health Protection Agency, which published the new figures, said one in 2,000 of the adult population of Britain were carriers of the condition, compared with one in 4,000 shown in a smaller survey in 2004.

Older people, born before 1961, were twice as likely to be carriers as younger people, yet less likely to develop the disease. Professor Sheila Bird of the Medical Research Council Biostatistics unit in Cambridge said: "Our dietary studies suggested older people were more exposed [to BSE infected meat] but they weren't turning up as clinical cases. They appear to be protected in some way. This shows how important it is to do the surveillance."

Professor Hugh Perry, chair of the MRC's neuroscience and mental health board, said: "These figures reinforce the importance that our efforts to prevent, diagnose and treat this devastating disease progress as rigorously as possible."

A Department of Health spokesman said: "These findings relate to people's potential to develop vCJD, not additional cases – in fact there have been no new UK cases for nearly two years.

"We have one of the safest blood supplies in the world, but experts will consider the Health Protection Agency study, and any additional measures to reduce the potential risk of transmission through blood transfusions will be put into place."