A haemophiliac contracted the human form of mad cow disease after being treated with a blood-clotting agent from an infected donor, experts said today.

The patient, who was over 70 and who died from an unrelated condition, is the first haemophiliac to be identified with vCJD.

The patient had shown no symptoms of the disease but a post-mortem revealed evidence of Creutzfeldt-Jakob Disease (vCJD) in the spleen.

Investigations are ongoing to work out how the patient contracted the disease, but he or she was treated with several batches of clotting factors before 1999 that were sourced in the UK.

Fears over mad cow disease and its possible transmission in blood transfusions led to tighter rules in 1999 around the supply and testing of blood.

The patient had been treated with one batch of Factor VIII - a protein that helps blood clot - taken from the plasma of a donor who went on to develop symptoms of vCJD six months after donating it in 1996.

The Health Protection Agency (HPA) said today that investigations were ongoing but that it was working with the UK Haemophilia Centre Doctors Organisation to inform all patients with bleeding disorders of the finding.

In 2004, all patients with bleeding disorders including haemophilia treated with UK-sourced pooled plasma products between 1980 and 2001 were classed as "at risk" of vCJD due to the possibility of infection.

The HPA said today that this latest discovery would not change the "at risk" status.

Professor Mike Catchpole, director of the HPA's centre for infections, said: "This new finding may indicate that what was until now a theoretical risk may be an actual risk to certain individuals who have received blood plasma products, although the risk could still be quite low.

"We recognise that this finding will be of concern for persons with haemophilia who will be awaiting the completion of the ongoing investigations and their interpretation.

"The priority is to ensure that patients are informed of this development and have access to the latest information and specialist advice from their own haemophilia centre doctor as soon as possible.

"This finding does not change our understanding of the risk from vCJD for other people in any specific way.

"But it does reinforce the importance of the precautionary measures that have been taken over the years.

"Since the risk of vCJD transmission through blood was first considered, a number of precautionary measures have been introduced to minimise the risk from the UK blood supply.

"UK plasma has not been used for the manufacture of clotting factors since 1999 and synthetic clotting factors are provided for all patients for whom they are suitable."