Patients who received blood products from donors who subsequently died from vCJD are to be warned they could be carrying the brain wasting disease.
Patients who received blood products prior to 1999 are at risk from vCJD
The measures are being taken after it emerged last year a patient had died following a transfusion from someone with the human form of mad cow disease.
The case was thought to be the first person-to-person vCJD transmission.
Only patients who received blood products before 1999 when new rules and procedures were introduced are at risk.
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, such as clotting factors, have been prepared from plasma imported from the US since 1998.
Health Secretary Dr John Reid said the new measures to prevent the possible spread of vCJD would include an assessment of blood products used before 1999 and informing anyone who received blood products from donors who later died of vCJD.
He said: "Many more patients of course, including haemophiliacs, will have received plasma products before plasma was sourced from the US.
"They will have received products derived from large pools of plasma donated from many thousands of people and thus heavily diluted.
"The UK-wide CJD Incidents Panel considers the risks for this group to be even lower than for those who received whole blood.
"It is very difficult to trace all individual recipients of products made from these plasma pools.
"However, the CJD Incidents Panel will be advising on a case-by-case basis which recipients will need to be contacted as the necessary information becomes
Patients will be informed in September after the Health Protection Agency has carried out the risk assessment.
It is understood letters will be sent urging each patient to contact a specialist.
But the Department of Health expressed concern that the results of the risk assessment exercise are communicated to patients in a sensitive and appropriate
manner directly by the doctors and other clinicians who care for them.
A spokesman added: "It would be grossly unfair on the patients involved to discuss the results of the risk assessment exercise in the media before patients
are informed of the outcome.
"A full public statement will be made at the appropriate time."
Any person in England and Wales with concerns may ring NHS Direct on 0845 850 9850.
Anyone in Scotland or Northern Ireland with concerns may ring NHS 24 on 0845 4242424.