Britain will become the first country in the world to create babies with the DNA of three people under government plans which could see the procedure offered on the NHS by next year.
Parents at high risk of having children with severe disabilities such as muscular dystrophy will be offered the controversial new IVF treatment after it was given the green light by ministers today.
It means the world's first "three-parent baby" could be born in Britain by 2015, if detailed proposals for regulating the procedure pass a public consultation and are approved by Parliament next year.
Up to 10 patients per year are expected to undergo the treatment, which involves replacing a fraction of the mother's damaged DNA with that of a healthy donor.
The process avoids the risk of the mother passing inherited defects, which can lead to a host of rare and debilitating conditions affecting the heart, muscles and brain, on to her children.
The technique is controversial because it involves "germ line" modification of the embryo's DNA, meaning the third party's genetic material would not only be passed on to the child, but also to future generations down the female line.
But ministers will publish draft regulations later this year allowing the therapy to "high-risk" families after a previous public consultation conducted by the Human Fertilisation and Embryology Authority last year revealed overall support.
The technique is aimed at tackling a collection of rare hereditary conditions which are caused by mutated mitochondria – structures which supply power to our cells.
About 99.8 per cent of our DNA, including all the genes which govern our appearance and identity, is found in the nucleus of cells and is inherited evenly from both parents, but a small fraction resides in the power-supplying mitochondria and is only passed on from mother to child.
Defects in the mitochondria can cause a range of serious problems including muscular dystrophy and affect about one in every 6,500 children born in the UK – greater than the number affected by childhood cancer.
So-called "mitochondrial replacement" therapy would avoid the risk of mothers transmitting such defects to their children, while still passing on the rest of their and their partner's characteristics.
Doctors would remove the nucleus from a donor egg and replace it with the equivalent genetic material from the mother's egg, either before or after fertilisation by the father.
The resulting egg could then be implanted and fertilised if necessary using standard IVF techniques. The child would inherit its identity from its mother and father but would gain their power-supplying mitochondria from the donor.
Prof Dame Sally Davies, the Chief Medical Officer, compared the process to changing a faulty battery in a car. The new mitochondria from the third parent make the child healthy but would not change their outward appearance.
"Mitochondrial disease can have a devastating impact on people who inherit it," she said. "It's only right that we look to introduce this life-saving treatment as soon as we can."
Professor Doug Turnbull, who pioneered the technique at Newcastle University, added: "I am delighted that the Government is moving forward with publishing draft regulations this year and a final version for debate in Parliament next year.
"This is excellent news for families with mitochondrial disease. This will give women how carry these diseased genes more reproductive choice and the opportunity to have children free of mitochondrial disease."
