Fears of a new wave of deaths caused by the human form of mad cow disease have been raised by a type of variant CJD not seen before.

Scientists who examined the brain of a 39-year-old woman killed by the disease found unusual patterns of damage.

The woman also had a distinct genetic make-up associated with her illness not shared by other victims.

Investigations are still at an early stage, but the findings point to the possibility of a new group of people succumbing to vCJD as a result of eating infected beef in the 1980s.

The disease, linked to misshapen prion proteins in the brain, has varying incubation periods according to genetic make-up.

Experts believe for many of those infected the timer could still be ticking. They might not develop symptoms for decades after acquiring the disease, or die from other causes before getting ill. In some cases the incubation period might exceed 50 years.

VCJD originally arose in cattle as bovine spongiform encephalopathy (BSE), or "mad cow disease", before appearing in a new form in humans. The incurable disease leaves the brain riddled with holes, like a sponge, causing a loss of mental faculties and death.

Since the disease was first recognised in 1995 there have been 162 confirmed or suspected deaths in the UK.

Fears of an epidemic involving hundreds of thousands of deaths abated after the number of annual deaths peaked at 28 in 2000 and then quickly fell. In the year up to November 30 2007 there were just four definite or probable cases. All these people had the MM version of the gene that makes prions, which is carried by about 40 per cent of the population.

The woman recently examined by scientists at the Medical Research Council's Prion Unit at University College London had a different gene variant, known as VV, found in about 10 per cent of Britons.