Tests at the UK's Institute of Animal Health indicated that one in six animals given blood from infected sheep appeared to develop the illness, the Guardian newspaper says.

The study suggests red cells and plasma may have infectivity for vCJD - the human form of BSE which destroys brain tissues - but the full version of the research is not published until November.

Scientists have previously demonstrated transmission of BSE in animals, but in animals fed infected brain.

The risk is very, very small in terms of catching vCJD through a blood transfusion

Professor Hugh Pennington

The National Blood Service said the research was valuable, but stressed there was no risk of contracting the disease by giving blood.

"We need to collect 10,000 units of blood a day to ensure lifesaving treatments can continue. Therefore it is vital that donors keep giving blood and that new donors keep coming forward," said a spokesman.

The study's authors suggest the risk of vCJD transmission through blood may be "appreciable" where the government had previously described it as "theoretical".

Precautions

The condition vCJD has claimed 114 victims in the UK, according to figures released in January, with experts predicting cases will increase by about 20% each year.

Professor Hugh Pennington, of Aberdeen University, said: "The worry is that there are more people out there incubating the disease.

"We don't know how many people are going to come down with vCJD."

He said even if there were thousands more carrying vCJD, the risk of catching it through a blood transfusion was "very, very small".

Prof Pennington said the transfusion risk was small

The government already has precautionary measures in place because of the theoretical risk of transmission via transfusion.

For example, it imports plasma from abroad as white cells are thought to be the element most likely to carry vCJD.

To date, there is no evidence of transmission from infected blood transfusions, even though 22 people have received transfusions from eight people who later contracted vCJD.

At present, there is no test for detecting vCJD in human blood. It is thought one may become available next year.

Expert advisors are expected to make new recommendations on blood transfusions within the next couple of months.

Future donors may be asked to take a test for vCJD before giving blood.

A Department of Health spokesman said: "These findings appear to justify the precautionary approach taken by the Department of Health to reduce the risk of vCJD transmission through blood and surgical instruments."