The brain disease variant CJD, triggered by eating BSE-infected meat, may have infected thousands more people than previously thought, scientists say.

Tests on tissue samples have shown that a wider group of the human population may be carrying the infection without showing symptoms and could transmit it via blood transfusions or contaminated surgical instruments.

The warning comes from researchers at the National CJD Surveillance Unit at the University of Edinburgh who checked more than 12,000 samples of tissue from tonsils and appendices removed in operations between 1996 and 1999. All the samples came from patients aged 20 to 29. Two samples harboured prion proteins - indicating vCJD infection - taken from people with a genetic make-up that had not previously been thought susceptible to the disease.

The finding raises the possibility that a second wave of cases could emerge years or even decades after the first if the incubation period in the new group of infected people is longer.

So far 161 people in the UK have succumbed to the incurable infection which slowly destroys the brain and leads to death. All cases so far identified have occurred in people carrying the methionine MM genotype, which is present in 40 per cent of the population.

The two new cases were found in tissue samples with the valine VV genotype, which is present in 10 per cent of the population. But no one with this genetic make-up has developed the disease, raising the possibility that they may be asymptomatic carriers who could pass it on via blood. The rest of the population carry the methionine/valine MV genotype.

Professor James Ironside, head of clinical neuropathology at the Edinburgh unit, said: "The fact that both these tissue specimens turned out to belong to the rarest of the genotypes in the population was surprising. It may be that they have a very long incubation period or never develop the disease but remain in a carrier state with the potential to infect others.

"People ask why, after widespread exposure to BSE in the 1980s, there have not been more cases of vCJD. The answer may be that more are infected but the infections haven't appeared yet or may never appear. The incubation period may be 40 to 50 years."

Estimates of the eventual size of the human epidemic have ranged up to hundreds of thousands of people infected but most scientists hoped that 10 years since the first human cases were identified, we were now over the worst.

The findings will increase pressure on the Government to change the rules for post-mortems to include tests for vCJD, which cannot be detected until after death.

Drug companies are racing to develop a screening test which could be used for blood donors to detect the presence of the disease before symptoms develop. However, this would raise ethical problems over the detection of a condition for the protection of others but which has no treatment.