The Department of Health will be asked tomorrow to license ground-breaking surgery involving a human being and a pig implanted with human genes.
The move will open the doors for pig-to-human organ transplants, but some scientists fear it could expose humans to a new range of animal viruses, triggering an epidemic comparable to Aids.
The move has shaken MPs and scientists who believed that such an animal- to-human operation was years away.
Department of Health officials are privately concerned that the surgery could set a precedent for the use of transgenic pigs as "spare part" producers for humans. They want the application to be minutely scrutinised.
"This is the first of its kind. It's a serious step. We are going to have to look at this procedure very, very carefully indeed," said a government source.
The pig-human procedure, which is expected to take place at a hospital specialising in transplants, would involve the insertion of genetically engineered tissue from a live pig in a human being.
One possible scenario would be the implanting of pig tissue into a human brain, as a treatment for conditions such as Parkinson's disease. The confidential application will be considered by a senior panel of advisers attached to the Department of Health.
The UK Xenotransplantation Interim Regulatory Authority, which is chaired by Lord Hapgood, the former Archbishop of York and a pharmacologist by training, was given new powers to vet applications last year by Frank Dobson, the Health Secretary. Ultimately, it is Mr Dobson who has the final say on whether to allow this advance. The committee will debate the ethics of the move and whether proposed safeguards - to make sure that pig viruses do not spread to humans - are sufficient.
Last year, Professor Robin Weiss, a viral oncologist at the Institute of Cancer Research, said that in the laboratory, two pig viruses had been found to replicate in human tissue.
The recipient of the pig tissue would have to be monitored for the rest of their life - and possibly quarantined if they did develop porcine diseases.
"The main dangers are the transfer of viruses from pigs to humans. There are no effective monitoring systems put in place," said a spokesman for the British Union for the Abolition of Vivisection.
"This is setting a dangerous precedent without a full public debate. Nothing like this has happened before. Many clinicians are worried about `Baboonisation', where cells of an animal migrate all over the human body. There are also no standards for protecting animals bred as donors."
Those in favour of such tissue transplants argue that they will be the answer to the shortage of organs for heart, liver or lung transplants which leads to the deaths of more than 200 people in Britain each year. In 1997 there were 5,732 people awaiting kidney transplants but only 1,635 were carried out.
Already in the US, operations have taken place to attach pig organs, such as a liver, outside the human body via a "bridge". And heart valves from ordinary pigs have been used in humans since the 1980s. But the new breed of British "humanised" pigs has never been used clinically before.
Church leaders and MPs believe such a move is "unethical" and that the results could be devastating.Reuse content