About 464 patients are receiving letters from doctors

The Department of Health said on Tuesday that it was writing to 464 patients to explain they were potentially at a small increased risk of infection.

However, Chief Medical Officer Dr Henrietta Campbell stressed "even if someone is identified as `at risk', it does not mean that they will develop vCJD".

"The thousands of people who have received a blood transfusion are not affected and have no cause for concern," she said.

It is that fear that you have a time bomb going off in your system - you don't know when, but you know it is ticking Nigel Hamilton

"It is relevant only to people who may have been treated with certain batches of some products made from plasma sourced in the UK. This small group of people are being informed."

Out of around 370 people who suffer from bleeding disorders, only a small number will have received implicated plasma products, she said.

Likewise, of the 94 people being contacted who have serious problems with their immune system, few, if any, would have received the implicated product, Dr Campbell said.

The action is part of a UK-wide patient notification exercise involving more than 6,000 people.

Dr Campbell said once a patient had been identified as being "at risk", they would be advised they do not need to take any precautions in normal life, but were being asked not to donate blood, tissues or organs.

A member of the Northern Ireland Haemophilia Society has said there is now a great feeling of fear and anxiety.

Nigel Hamilton who is a haemophiliac said: "There is an unknown factor with vCJD.

"You could find yourself with vCJD contaminants in your blood, but no-one is in a position to tell you when it is likely to strike, and how rapidly," he said.

"It is that fear that you have a time bomb going off in your system - you don't know when, but you know it is ticking."

The nationwide warning exercise was organised following the first possible case of a person dying after contracting vCJD through a blood transfusion, announced by Health Secretary Dr John Reid last December.

Transfusion ban

This led to a ban on people who have received a blood transfusion since January 1980 donating blood in the future.

Then in July it emerged that another patient who received donated blood was found to be carrying the vCJD agent, though they died of an unrelated cause.

Dr Angela Robinson, from the National Blood Service , said they had identified nine donors in the UK who went on to develop vCJD.

Together, the donors made 23 plasma donations which contributed to about 200 batches of plasma.