The government says the risk is small and that it is taking a "highly precautionary" stance in telling the patients of the possible risk.

"These patients (and their healthcare professionals) are being advised that they have in the past received batches of plasma products which were derived from blood donated from someone who has later gone on to develop vCJD," says a Department of Health statement.

Those most likely to be affected are approximately 4,000 people with hemophilia and other bleeding disorders; about 50 people with primary immunodeficiency; and a small number of people who have been treated with large quantities of particular plasma products.

It is not possible to know exact numbers of people in any of these groups until the patient records have been examined, a process that began on September 9, the statement says.

It suggests a number of steps to reduce any possible onward patient-to-patient transmission of the disease: not donating blood, tissue or organs, and telling doctors and dentists who may be treating them.

The situation developed due to two incidents where vCJD is suspected to have been passed on by blood transfusion, dating to December of last year.

Blood donated by a small number of people who went on to develop vCJD has been traced. People who received direct, one-to-one transfusion of "whole blood" from those donors were contacted earlier this year and told about any additional risk they may face.

Now plasma from these same donors used to manufacture clotting agents and other products also has been traced. Plasma products are manufactured from pools of many thousands of donations, greatly reducing any risk of vCJD being passed on.

"Throughout our handling of the issue of vCJD, we have adopted a highly precautionary approach," Chief Medical Officer Sir Liam Donaldson said, "taking a series of steps as new evidence became available to maximise the protection of the public."

Health Secretary John Reid acknowledged that "this information may be difficult to absorb, which is why we are working with their doctors and other clinicians, to ensure they have the information and support they need."