Scientists given go-ahead on creating cow-human embryos to find cures for Alzheimer's and leukaemia

King's College will now fuse eggs from the animal with human skin cells.

Researchers hope to make an "unlimited" source of stem cells, which they believe could be used to help find cures for diseases including Parkinson's, Alzheimer's and leukaemia.

Ethics watchdog the Human Fertilisation and Embryology Authority allowed the research despite opposition from "pro-life" groups.

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It said the benefits outweighed the risks and the public was at ease with the idea.

The embryos will be more than 99 per cent human. They must be destroyed after 14 days to ensure there is no chance that one is implanted in a human womb.

Researchers have been waiting to start work since November 2006 but the decision was delayed amid calls for a public consultation. A parallel project will start at Newcastle University.

Dr Stephen Minger, who will run the King's College project, said: "After a year and a half (the watchdog) has realised the importance of the work."

Opponents pledged to go to the courts. John Smeaton, national director of the Society for the Protection of Children, said: "This represents a disastrous-setback for human dignity.

"The deliberate blurring of the boundaries between humans and other species is wrong and strikes at the heart of what makes us human.

"It is creating a category of beings regarded as sub-human who can be used as raw material to benefit other members of the human family, effectively creating a new class of slaves."

Josephine Quintavalle of Comment on Reproductive Ethics said: "This work is highly unnatural and highly dubious in terms of safety."

Scientists will take genetic material from cow eggs and replace it with DNA from human cells to make pinheadsized "cytoplasmic embryos".

These hybrids are packed with stem cells - the body's "master cells" which can potentially develop into any type of human tissue - making them perfect for research.

In the far future scientists may be able to grow new tissue in the laboratory.

Dr Lyle Armstrong of Newcastle University said: "This is a fantastic day for medical research. We want to use the embryonic stem cells.

"We could reprogramme cells to treat diseases such as Parkinson's, leukemia or muscular dystrophy. The list is endless."

The HFEA said: "The two applications satisfied all the requirements of the law. We have now offered one-year research licences to the two applicants, subject to a series of detailed conditions in each case."

Professor Robin Lovell-Badge, head of stem cell biology at the National Institute for Medical Research, said: "The HFEA has been delaying this decision for months because they have been nervous of public reaction. It is a great relief that they have finally said yes."