Thousands of people are being warned they may have been exposed to vCJD through contaminated blood products.
Concerns were first raised over blood transfusions
The CJD Incidents Panel says about 4,000 in the UK, mostly haemophiliacs, are thought to be at risk.
The patients may have received blood products, such as clotting agents, from donors who subsequently developed vCJD.
But the government is stressing the risk is very low, and relates to people exposed to blood products before 1999 when precautions were brought in.
Nine donors have been identified as having developed vCJD after giving blood.
Their donations would have gone into plasma pools that would have been diluted by thousands of other donations.
The CJD Incidents Panel said this diluting would reduce the risk of contamination.
In total, 6,000 letters are to be sent out informing patients of the risk assessment - although only two thirds are being told they may be at direct risk.
There is no test for the brain wasting disease so those at risk have no way of knowing whether they have vCJD.
As well as haemophiliacs, a small number of people with immune system disorders will receive the letters.
Government scientists say more work is needed to determine the exact risk each individual has been exposed to.
But those at risk will be told not to donate blood, and to inform doctors and dentists if they undergo treatment in the future in a bid to stop further onward transmission.
Professor Don Jeffries, acting chair of the CJD Incidents Panel, said: "In the interests of public safety, we have been advised to take the most precautionary measure."
Sir Liam Donaldson, the chief medical officer, said the risk needed to be looked at in context of the exposure the public has had to BSE in the 1980s and 1990s through meat.
"The entire population of the UK was exposed to a massive risk of vCJD through eating contaminated BSE products."
And Professor Noel Gill, of the Health Protection Agency, which is coordinating the informing of patients, said: "The bottom line is that we are taking this precaution to minimise patient-to-patient transmission.
"The price of that is that a number of patients get a letter saying they could be at risk.
"This is clearly quite an overwhelming piece of information to come to them."
Health Secretary John Reid launched the review of vCJD infection risk in December after announcing that a person had died from the human form of BSE after receiving blood from a donor who later developed the disease.
A second person who had received a blood transfusion has subsequently died from other causes but had signs of vCJD in the spleen.
The government has already identified 17 people who received blood transfusions from people who went on to develop vCJD.
Since 1997 all cases of vCJD that are reported to the National CJD Surveillance Unit and diagnosed as having "probable" vCJD are passed on to the National Blood Service which searches its blood donor records.
If the patient has given blood, subsequently any stocks of that blood are immediately destroyed.
White blood cells, which it is thought may carry the greatest risk of transmitting the disease, have been removed from all blood used for transfusion since 1999.
And blood products have been prepared from plasma imported from the US since 1998.
Graham Whitehead, the chief executive of the Haemophilia Society, said the news would cause anxiety, especially in the wake of HIV and hepatitis C infection from blood products.
But he said it must be put in context. "It is very reassuring that no-one in the world with haemophilia has been diagnosed with vCJD."
The Liberal Democrats are calling for an inquiry into contamination of blood products at their conference this week.
Bruce Norval, a haemophiliac from Fortrose, near Inverness in Scotland, said: "Ever since vCJD came about I thought this could happen.
"It is just like HIV and hepatitis C all over again, I now face a lifetime of fear watching for signs of the illness."
Any person with concerns can ring a dedicated NHS Direct helpline on 0845 850 9850 for England and Wales, and NHS 24 on 0845 4242424 in Scotland and Northern Ireland.