But researchers at Imperial College London believe only about 70 future cases of variant Creutzfeldt-Jakob disease (vCJD) will be diagnosed in Britain.

"We think that the epidemic will be quite small in terms of cases that have arisen from consumption of beef," said epidemiologist Dr Azra Ghani.

The calculations are based on tonsil and appendix samples, where evidence of infection is most evident, taken from 12,764 people. Three samples tested positive but only one matched tissue taken from a person with the disease.

The pool of infected people could be up to 3,800 people, but only a small number of them would develop the illness.

"One reason for the discrepancy between the high estimated number of positive tests and low number of actual recorded clinical cases could be that many infected individuals do not go on to develop clinical disease in their lifetime," Ghani said.

Because of the long incubation period, which scientists estimate could be from 10 to 20 years, it has been difficult to predict how many cases of vCJD there will be.

Up to Nov. 1 last year, 146 people had died from definitive or probable vCJD in Britain, the Department of Health says.

Ghani and her team reported their findings in the Journal of the Royal Society Interface. Their estimate only refers to people infected through BSE-infected beef.

"Although our results indicate there is little chance of large numbers of vCJD infections from primary transmission, we have not taken into account the possibility of additional cases infected by blood transfusion. This could result in more clinical cases emerging at a later date," she said.

Two suspected cases of vCJD via blood transfusion have been reported so far. The government announced what was thought to be the world's first case in December 2003 after a patient died several years after receiving blood from a donor later found to have had the illness.

The second case was reported last July. The patient did not die of vCJD but an autopsy of the individual, who had a blood transfusion five year earlier from a person who later developed vCJD, showed the infectious agent in the spleen.