BBC Science Correspondent James Wilkinson: "There are ethical questions about the research."

American researchers have managed to isolate and culture embryonic stem cells. These are the undeveloped "parents" of all the tissues in the body with the potential to become anything from skin and bone to muscle and blood.

It might now be possible, with many more years of research, to grow specific tissues and even organs for use in transplants.

But the methods used by the scientists have angered pro-life campaigners. One of the research teams isolated their stem cells from early stage fertilised embryos left over from IVF treatment. The other team, using a different technique, cultured the cells from the tissue of aborted foetuses.

Anti-abortion groups fear the technology will be combined with cloning

"It is morally unacceptable because it involves the deliberate destruction of a human being," said Professor Jack Scarisbrick, national chairman of anti-abortion chairty Life.

"It's a form of cannibalism. We're not against medical advances but if you go down this path you end up in moral chaos. We don't need to go down this hi-tech Brave New World route."

But the fertility expert Lord Winston rejected the criticism and welcomed the benefits he believed would now come in the next few years and decades.

"This is a marvellous development," he said. "It's one that will protect and enhance human life and it would surely be unethical not to do it."

Cloning techniques

Those commentators worried about the pace of scientific development are concerned that the new technology might be combined with the cloning techniques that gave us Dolly the Sheep - methods they have already said are ethically unacceptable.

"Theoretically you could encourage stem cells to produce an unlimited number of sperm or egg cells which could be engineered to contain supergenes and put on the market," said Dr Patrick Dixon, author of Futurewise, which warns of the dangers of unchecked science.

Karen Lebacqz: Geron does not want to pursue cloning

"They'd have enhanced physical, mental or other characteristics which would be carried on to the next generation. You could also take stem cells from the embryo, re-programme them, and then re-implant them so you end up with a genetically altered embryo."

However, Karen Lebacqz from the Geron Corporation, which funded the stem cell research, said cloning was not being considered by her organisation.

"We would not support the effort to use any of this research to do cloning with the intent to produce a human child or to inject these cells into another developing embryo in order to create a human being that has this mixed genetic composition," she said.

The Human Fertilisation and Embryology Authority which governs fertility treatment and research in the UK said it was questionable whether the American research would be allowed in Britain under the current law.

For research involving the use of human embryos to be licensed, it has to meet certain legal criteria which do not quite fit what the Americans are doing.