The announcement follows news that a British biotechnology company - Imutran UK - wants to give a pig's liver to a patient suffering from organ failure. The pig liver would be used to keep the person alive until a suitable human donor could be found.

Pigs have been engineered for use in transplants

Even if the technical problems of xenotransplantation can be overcome, some scientists believe the procedure may be dangerous. They fear animal viruses could be passed to the patient and then spread to the wider population.

Mr Dobson said any operation had to be safe before it could go ahead.

"Trials in xenotransplantation involving humans will only be allowed to take place if and when we are fully satisfied that the risks associated with such procedures are acceptable taking account of all the available evidence at the time," he said.

His guidelines are a response to a consultation exercise on the issue begun last year.

These are some of the areas that are covered:

• Hospitals must follow set procedures if they want to do an operation.
• The UK Xenotransplantation Interim Regulatory Authority (UKXIRA) will assess the applications and advise ministers who will then take the final decision.
• The welfare of animals used for xenotransplants is to be given a high priority. There will be new guidelines to ensure they are looked after properly and everything is done to ensure they do not become infected by pathogens that might endanger human health.
• There will be an international meeting of scientists to discuss the pig viruses which could conceivably jump to human beings.
• A working group is going to look at the infection risks of the different types of xenotransplantation therapy being proposed.
• The UKXIRA has put up a Website to publicise its work. It can be found at www.open.gov.uk/doh/ukxira.htm.

Mr Dobson said the regulation of xenotransplantation would have statutory backing.

Animal welfare is a top priority

He stressed the government's view that Great Apes - chimpanzees, gorillas and orang-utans - should not be used in any xenotransplant because they were the animals most likely to harbour viruses which could jump the species barrier.