Scientists say there is "an appreciable risk" of people catching the human form of BSE through blood transfusions, a significant upgrading of the threat posed by the inevitably fatal disease.
The results from tests on sheep, seen by the Guardian newspaper, suggest the danger is far more serious than first suspected. Previously the risk has been described by the government as "theoretical".
One in six animals given blood from infected sheep appears to have caught the disease so far.
The experiments indicate that more blood components than previously thought might carry the killer agents of BSE and variant CJD, and the blood might be infective long before animals, and by extension humans, show outward signs of the long-incubating diseases.
The findings, circulated to blood specialists, are regarded as enormously significant. They have prompted urgent consideration of new measures to protect the public and orders to hospitals to drastically reduce their use of blood, including using technology that allows the recycling of patients' own blood.
The British Department of Health and blood transfusion services last night appealed to the public to continue donating blood. A spokesman for the department said: "These findings appear to justify the precautionary approach taken by the department to reduce the risk of vCJD transmission through blood and surgical instruments.
"Our assessment has been that the blood of variant CJD patients may contain transmissible levels of infectivity before clinical signs are apparent."
So far 115 Britons have died and nine others are thought to be dying from vCJD, for which the prime culprit still appears contaminated meat eaten many years ago.
The government has introduced precautionary measures over blood but it considered the risk theoretical even after autumn 2000, when scientists in Edinburgh and Berkshire revealed that blood from a sheep fed cattle brain infected with BSE had infected a previously healthy animal.
All blood given to humans has the white cells removed because they are regarded as potentially the most dangerous carriers of vCJD through transfusion. Most plasma is imported because of risks to haemophiliacs who have a constant need for the blood clotting factors.
The government is considering banning all people who have ever received transfusions from donating blood, a decision that might remove one in 10 donors; importing more plasma, particularly for transfusions in babies and young children, and forcing the removal of plasma from blood platelets.
The possibility of a blood test for vCJD next year, though essential for public safety, might dissuade up to half Britain's donors, because people would have to face being told they have an incurable disease. So far there is no evidence that anyone infected with vCJD has passed it on to others through transfusion. - (Guardian Service)