The Health Protection Agency has confirmed the first case of the human form of "mad cow disease" in a patient with haemophilia.
A post-mortem showed the 70-year-old man, who had received blood products before strict rules were introduced to limit contagion, died infected.
However he died of other causes and had not shown any symptoms, the HPA said.
Up to 4,000 haemophilia sufferers have been warned they could be at risk of variant Creutzfeldt-Jakob disease.
The HPA says it is still unclear how the man contracted the disease, but it is known that he was treated with several batches of UK blood products before 1999, when the risk of transmission was not known.
Specifically, it is thought he was treated with a batch that was manufactured from the plasma of a donor who went on to develop symptoms of vCJD.
It is the first time that vCJD has been found in one of the thousands of haemophiliac patients who received these blood plasma transfusions before 1999. They have been told they are all run a "low risk" of developing the disease.
"This new finding may indicate that what was until now a theoretical risk may be an actual risk to certain individuals who have received blood plasma products, although the risk could still be quite low," said Professor Mike Catchpole of the agency's Centre for Infections.
"We recognise that this finding will be of concern for persons with haemophilia who will be awaiting the completion of the ongoing investigations and their interpretation."
There is no test that can screen for vCJD, although development work is underway. However, even if an effective test were available, there is no means of treating the condition.
Professor Catchpole added: "This finding does not change our understanding of the risk from vCJD for other people in any specific way. But it does reinforce the importance of the precautionary measures that have been taken over the years."
UK plasma is no longer used for the production of the clotting treatment haemophiliacs require, and when suitable, synthetic products are used. THE BBC