Scientists believe they have made a potential breakthrough in the treatment of serious disease by creating a human embryo with three separate parents.
The scientists have created the embryo in the lab
The Newcastle University team believe the technique could help to eradicate a whole class of hereditary diseases, including some forms of epilepsy.
The embryos have been created using DNA from a man and two women in lab tests.
It could ensure women with genetic defects do not pass the diseases on to their children.
The technique is intended to help women with diseases of the mitochondria - mini organelles that are found within individual cells.
They are sometimes described as "cellular power plants" because they generate most of the cell's energy.
Faults in the mitochondrial DNA can cause around 50 known diseases, some of which lead to disability and death.
About one in every 6,500 people is affected by such conditions, which include fatal liver failure, stroke-like episodes, blindness, muscular dystrophy, diabetes and deafness.
At present, no treatment for mitochondrial diseases exists.
The Newcastle team have effectively given the embryos a mitochondria transplant.
They experimented on 10 severely abnormal embryos left over from traditional fertility treatment.
Within hours of their creation, the nucleus, containing DNA from the mother and father, was removed from the embryo, and implanted into a donor egg whose DNA had been largely removed.
The only genetic information remaining from the donor egg was the tiny bit that controls production of mitochondria - around 16,000 of the 3billion component parts that make up the human genome.
The embryos then began to develop normally, but were destroyed within six days.
Experiments using mice have shown that the offspring with the new mitochondria carry no information that defines any human attributes.
So while any baby born through this method would have genetic elements from three people, the nuclear DNA that influences appearance and other characteristics would not come from the woman providing the donor egg.
However, the team only have permission to carry out the lab experiments and as yet this would not be allowed to be offered as a treatment.
Professor Patrick Chinnery, a member of the Newcastle team, said: "We believe that from this work, and work we have done on other animals that in principle we could develop this technique and offer treatment in the forseeable future that will give families some hope of avoiding passing these diseases to their children."
Dr Marita Pohlschmidt, of the Muscular Dystrophy Campaign, which has funded the Newcastle research, was confident it would lead to a badly needed breakthrough in treatment.
"Mitochondrial myopathies are a group of complex and severe diseases," she said.
"This can make it very difficult for clinicians to provide genetic counselling and give patients an accurate prognosis."
However, the Newcastle work has attracted opposition.
Josephine Quintavalle, of the pro-life group Comment on Reproductive Ethics, said it was "risky, dangerous" and a step towards "designer babies".
"It is human beings they are experimenting with," she said.
"We should not be messing around with the building blocks of life."
Mrs Quintavalle said embryo research in the US using DNA from one man and two women was discontinued because of the "huge abnormalities" in some cases.
Dr David King, of Human Genetics Alert, expressed concern about a "drift towards GM babies".