HOPES that the worst of the human "mad cow disease" nightmare is over are today dashed by scientists who warn that Britain could still face an epidemic.

New evidence published in The Lancet suggests that many more people may be susceptible to variant CJD than was previously supposed.

The findings emerge from the case of a patient believed to have contracted the human form of BSE through a blood transfusion.

The patient's genetic make-up is different from that of any known previous victim of the disease.

With this new group of potential victims, the incubation period of the disease may be much longer - raising the possibility of a major epidemic of cases occurring at some unknown time in the future.

Meanwhile, there could be a large pool of symptomless people who do not realise they are carrying the infectious prion protein responsible for both the cattle brain disease BSE and vCJD.

There is a chance they could transmit the agent to others via blood transfusions or contaminated surgical instruments.

The total number of deaths in the UK definitely or probably caused by vCJD now stands at 147. Most of these victims are thought to have acquired vCJD by eating infected beef in the 1980s.

Cases of vCJD peaked in 2000, when 28 deaths were reported. Since then the trend has been generally downward, with 18 last year.

But the genetic discovery reported today in The Lancet medical journal suggests it would be wrong to assume the danger is passing.

Every previous vCJD victim examined by doctors has had a similar genetic make-up. As a result it has been assumed that only these individuals, representing about 37 per cent of the population, are susceptible.

But the blood transfusion case described today has a genetic make-up found in about half the population.

Professor James Ironside, from the National CJD Surveillance Unit in Edinburgh, who took part in the study, said: "It's absolutely possible that there may be a new epidemic, because the cases we've seen so far may only be those who are unusually susceptible or have the shortest incubation periods.

"I'm not in the business of scaremongering, but quite clearly the idea that this problem is on the way out is unfortunately not the case at all."

The patient, whose age and sex have not been revealed, died from a ruptured artery five years after receiving blood from a donor who later developed vCJD.

It was the second case of a patient appearing to have picked up the disease from a blood transfusion.

The first was diagnosed with vCJD last year. That case prompted an investigation which identified 17 living individuals who had received blood components from donors who succumbed to vCJD.

After the second case came to light, the Department of Health said it was extending measures to limit the number of blood transfusion recipients who can become donors.

A second paper in The Lancet raises further doubts about the safety of blood transfusions.

The study suggests that the system of removing white blood cells to reduce the risk of vCJD transmission may not be as effective as hoped.

Frances Hall, of Chester-le-Street, County Durham, is secretary of the Human BSE Foundation. Mrs Hall, whose son, Peter, died from the illness in 1996, said: "The hope was that only those of an unusual genetic type would develop vCJD. Unfortunately it now looks like more people could be susceptible.