This is a major setback for doctors who hope that animal to human transplantation is the answer to donor organ shortages.

The man, a 35-year-old HIV patient, died from liver disease just two months after the transplant.

But tests after his death revealed that a herpes virus known as cytomegalovirus (CMV) - which is present in virtually all wild baboons - had crossed the species barrier.

Marian Michaels, of the University of Pittsburgh, said: "This is the first time that a virus has actually been cultured from a person who received an animal transplant."

The patient was given antibiotics and antiviral drugs when given the baboon liver.

'Quite concerning'

Experts had hoped that the baboon virus would not be able to survive in a human.

Ms Michaels said: "I think it is quite concerning that an animal virus thought to be species-specific could be transmitted."

The patient died shortly after the operation

She told a meeting of the American Society for Microbiology that the finding struck a blow to the idea that primates could be used as a source of organs for transplantation.

However, she said one 25-year-old Aids patient had received bone marrow from a baboon which had been quarantined from other animals from birth, and was therefore free of the infections naturally acquired in primate communities.

But this option raised difficult ethical issues involving the humane treatment of animals.

Operations halted

Because of the risk of unknown infections - which could potentially have far more devastating effects on humans than on the animal - US authorities have called a halt to such transplants for the time being.

The Council of Europe has also ordered a moratorium on animal to human transplantation, or xenotransplantation, because of the risks of infection.

Much work has focused on transplanting organs from genetically modified pigs into humans, but no such operations have been carried out in the UK.

Xenotransplantation is seen as a possible solution to a chronic shortage of donor organs worldwide.

Thousands of people are awaiting liver and heart transplants in the UK.