Thousands of patients have received letters saying they may be at risk of developing the human form of “mad cow disease”. Health Secretary John Reid said the patients may have been exposed through blood plasma products.About 4,000 people may be at a very small risk of developing vCJD.

The Chief Medical Officer, Sir Liam Donaldson, said he was adopting a “highly precautionary approach”.

He said the risk of these patients developing vCJD was very small.

The warnings come after the first possible case of a person dying after contracting vCJD through a blood transfusion was announced by Dr Reid last December.

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

It later emerged that another patient who received donated blood was carrying the vCJD agent – although they died of unrelated causes.

A risk assessment was ordered into blood plasma products, ie. those derived from donated blood and used in products such as clotting agents.

The Department of Health said 6,000 potentially exposed people would receive letters telling them about the risk assessment exercise.

The number who may be affected directly was thought to be around 4,000 people.