Britain extended restrictions on blood donations on Thursday after a second person was suspected of being infected with the human form of mad cow disease through a blood transfusion.

The government announced what was thought to be the world’s first case of transmission of variant Creutzfeldt-Jakob disease (vCJD) via transfusion last December, after a patient died several years after receiving blood from a donor later found to have had the illness.

“A second case of possible transmission of vCJD from person to person via blood transfusion has now been confirmed by the National CJD Surveillance Unit,” the Department of Health said in a statement on its Web site.

It added that the unidentified patient, now dead, received a blood transfusion in 1999 from a donor who later went on to develop vCJD.

Secretary of Health John Reid said the government was taking a precautionary approach and added that people should continue to have a blood transfusion when it is necessary.

“Although people may have concerns about the implications of this announcement, I would emphasize again that the exclusion criteria are being tightened because of a small but unquantifiable risk,” he said in a statement.

After the first suspected case, Britain banned people who have had transfusions over the past 24 years from donating blood, to reduce the risk of spreading vCJD.

It widened the restrictions on Thursday to include donors who are unsure whether they have previously had a blood transfusion and those only donating a specific blood component such as platelets.

Variant CJD is the human equivalent of bovine spongiform encephalopathy (BSE) or mad cow disease, an incurable, degenerative brain disorder linked to eating meat infected with BSE.

The illnesses are caused by normal brain proteins, called prions, that transform themselves into infectious agents.

The second patient did not die of vCJD but an autopsy showed the presence of the infectious agent in the patient’s spleen.

Fifteen people in Britain have received donations of blood from donors who subsequently developed vCJD. All were contacted, informed of the risks and counseled. The earliest transfusion was in 1993 and the latest in 2001.