A UK patient who died from CJD may be the first in the world to have caught the illness from a blood transfusion.
Health Secretary John Reid said that a patient who received donor blood during an operation in 1997 developed variant CJD and died six years later.
The blood was taken long before the donor was diagnosed with the brain-wasting disease.
Measures already exist which attempt to cut the risk of CJD transmission during blood transfusions.
So far, 143 cases of vCJD have been diagnosed in the UK , although the numbers of new cases are falling.
So far there is no established treatment for the illness, which causes massive brain damage and normally kills within months of being detected.
Incubation period
Other people have received blood from donors who went on to develop vCJD - 15 in total, two of which live in Scotland.
All have been contacted and offered counselling.
However, none of these have so far gone on to develop the disease themselves, although it may have a long incubation period.
The risks of receiving blood carrying the "rogue" prions which cause vCJD are largely unknown, although previously thought to be tiny, as no confirmed cases could be identified.
In this case, the donor involved gave blood in 1997, and fell ill with CJD in 1999, dying shortly afterwards.
The disease did not develop in the recipient until this year, and the patient died earlier this month. Post mortem results appear to confirm vCJD.
Mr Reid, in a statement to the House of Commons, said: "This is possibly not a proven causal connection - it's also possible that both individuals acquired CJD separately.
"This is a single incident, so it is impossible to be sure which was the
route of the infection.
"However, the possibility of this being transfusion-related cannot be
discounted.
"That is the conclusion of the Chief Medical Officer and experts.
"It is because this is the first report from anywhere in the world of the
possible transmission of vCJD from person to person via blood that I thought it
right to come to the despatch box to inform the House on a precautionary
basis."
However, he conceded that there was a chance this was the first recorded case of blood from an apparently healthy donor causing CJD in the recipient.
One other person is thought to have received blood from the same donor.
Precautions
The announcement is likely to cause concern among the tens of thousands of patients who have received blood transfusions each year.
The concern is that, because of the long incubation period of vCJD in humans, that other regular donors might be carrying the illness without knowing it - or having any way of finding out.
However, since this transfusion, stringent measures have been introduced in an effort to make blood taken from UK donors safer.
The white cells from the blood - thought to be more likely to harbour prions - are routinely removed from UK donations.
In addition, many blood products used in the UK are manufactured using donated blood from elsewhere in the world.
The US banned the use of UK donor blood when fears over vCJD first arose.
Mr Reid has asked the government's expert committee on blood to urgently examine whether new measures are needed to ensure the safety of donated blood.
He has also asked the National Blood Service to enter discussions with the medical royal colleges and NHS hospitals to ensure blood products are only used when they are only absolutely necessary.