Scientists will be able to carry out genetic experiments on human embryos for the first time under controversial Government plans.
Ministers have given the green light to research that some doctors warn raises the disturbing prospect of "genetically modified babies".
Proposed legislation says researchers would be allowed to alter the genetic blueprint of an embryo up to two weeks old. Ethical campaigners fear this could herald an era of eugenics in which couples would engineer their children to be as perfect as possible.
Critics predicted the public would be "horrified" by any steps towards creating GM babies.
The Government has said it will not allow genetic modification of embryos for reproductive purposes.
However, it wants to let scientists alter the genetic structure of the cells of live embryos for research projects. In practice this would mean scientists could take an embryo while it is only a few days old and manipulate its genes to learn more about inherited disorders such as motor neurone disease.
After 14 days the embryo would have to be destroyed so it could never grow into a GM baby.
But Dr David King, director of the campaign group Human Genetics Alert, said that to allow such research suggests that one day the Government hopes to overturn this current ban.
"If this were not the plan, why allow scientists to begin research?" he asked. "We must not start down the path to a future of GM "designer babies".
"Once scientists can dangle before the public realistic rather than theoretical possibilities of curing genetic diseases, it will be difficult to counter well-meaning support for human genetic modification."
He said current techniques are unsafe and would be likely to create mutations in the embryos.
Genetic modification could ultimately be used to improve a potential child's character or appearance, he added. "Then we could end up in eugenics that would make anything else we have seen in the past look like a picnic in the park."
Josephine Quintavalle, of the lobby group Comment on Reproductive Ethics, said: "This is very clearly a step in a direction that no one should be going down.
"There was enough concern about genetically-modified food so what will people think of the genetic modification of human beings?"
Dr Michael Antoniou, an expert in gene therapy from Guy's Hospital in London, said other techniques already exist to help couples carrying defective genes.
"If we really had the good of the patient at heart we would be investing our limited resources in this area to advancing gene therapy in combination with adult stem cell technology which is already showing great promise in clinical trials," he said.
But others disputed the idea that research on 14-day-old embryos would lead to GM babies.
Professor Chris Higgins, director of the Medical Research Council's Clinical Sciences Centre and an expert in genetics, said scientists want to understand more about the early stages of human development and disease.
Such embryos could be used to create banks of stem cells, which scientists could then use to study ways in which diseases form and even try out new treatments.
"We need to learn how to do this perfectly and to be able to do that there are fundamental questions that have to be answered about how cells work in the embryo," he said.
He said that at the stage when the embryo is destroyed it is a bundle of 100 cells. It is not yet a baby because it has not been implanted in the womb - and that would be banned under current laws.
"It is completely illegal to use these techniques to create a designer baby," he said. "There are very clear safeguards in this country and I don't know of anyone who would even think of doing that as it would be unsafe and immoral."
Professor John Burn, medical director of the International Centre for Life, Newcastle, said: "I can't imagine anyone wanting to use genetic modification in reproduction in the foreseeable future."
Dr Kieran Breen, director of research and development for the Parkinson's Disease Society, urged the Government "not to cut off a research channel that offers the 120,000 people with Parkinson's in the UK significant - but as yet not fully explored - hope for the future."
The Department of Health said: "The law will clearly continue to ban genetic modification of embryos for reproductive purposes and we will extend that prohibition to explicitly cover sperm and eggs.
"Any research using human embryos requires authorisation from the Human Fertilisation and Embryology Authority and must meet the rigorous criteria set out in the Human Fertilisation and Embryology Act 1990."
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