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To make a hybrid embryo, a human skin cell would be taken from a person with motor neurone disease and injected into a hollowed-out rabbit egg. The resulting embryo would contain only a tiny amount of rabbit DNA in a microscopic structure that generates energy in the cell. The rest of the DNA would be human. If the experiment is successful, within a week, the egg will have divided to form a tiny ball of a 200 or so cells, from which stem cells could be extracted.

The embryos could not legally be implanted into a woman's womb and the stem cells would not be safe to implant because they would be rejected by the immune system. 'They will never grow beyond the 200 cell stage and they will have no human features,' said Prof Shaw.

The proposal exposes a grey area in British regulation, however, as officials at the HFEA admitted it was questionable whether the resulting embryo was human. 'That's the question and it's for the government, the HFEA and lawyers to work out,' said Prof Shaw.

Josephine Quintavalle of the lobby group Comment on Reproductive Ethics said: 'There is a lot of innate wisdom in the yuk factor, or repugnance as it is also known. My question is: what will they actually create? It is simplistic or deliberately deceptive to say they are simply making stem cells. In order to obtain stem cells they surely have to go through the blastocyst stage; they have to create a 'something' from which to derive the new cells. What is this something? It must be human to be of any use to researchers.'

Professor Sir John Gurdon, a Cambridge University researcher, already uses similar technology to investigate how eggs appear to be capable of converting adult cells into stem cells that can potentially grow into any tissue in the body. His experiments have so far focused on injecting DNA from human cells into frog eggs.

He said: "I don't see there's any ethical problem with what they are proposing. I don't see it as a human embryo, but it all comes back to the question of when you think life begins. Scientifically, though, I'm not persuaded it will work. If you put cells from one species into the egg of another, the egg may divide, but you could get a lot of genetic abnormality that won't lead to good quality stem cells."