* Britain would be first to offer the treatments
* Technique involves implanting genetically modified embryos
* Govt, medics say aim is to prevent devastating diseases
* Critics fear slippery slope to eugenic designer babies (Adds comment from doctor, criticism from genetics campaign group)
By Kate Kelland, Health and Science Correspondent
LONDON, Feb 27 (Reuters) - Britain proposed new regulations on Thursday that would make it the first country in the world to offer "three-parent" fertility treatments to families who want to avoid passing on incurable diseases to their children.
The move was praised by doctors and but feared by critics, who say the technique will lead to the creation of genetically modified designer babies.
The technique is known as three-parent in vitro fertilisation (IVF) because the offspring would have genes from a mother, a father and from a female donor.
The British plans come as medical advisers in the United States began a series of public hearings this week to consider whether there is scientific justification for allowing human trials of the technique.
The treatment, only at the research stage in laboratories in Britain and the United States, would for the first time involve implanting genetically modified embryos into women.
The process involves intervening in the fertilisation process to remove faulty mitochondrial DNA, which can cause inherited conditions such as fatal heart problems, liver failure, brain disorders, blindness and muscular dystrophy.
It is designed to help families with mitochondrial diseases - incurable conditions passed down the maternal line that affect around one in 6,500 children worldwide. Mitochondria act as tiny energy-generating batteries inside cells.
Announcing draft plans to allow the technique and launching a public consultation on them, Britain's chief medical officer Sally Davies said the proposed move would give women who carry severe mitochondrial disease the chance to have children without passing on devastating genetic disorders.
"It would also keep the UK in the forefront of scientific development in this area," she said in a statement.
But David King of the campaign group Human Genetics Alert accused the government of "jumping the gun" in laying out new laws before the treatments had been thoroughly investigated.
"If passed, this will be the first time any government has legalised inheritable human genome modification, something that is banned in all other European countries," he said in a statement. "Such a decision of major historical significance requires a much more extensive public debate."
Although some critics of mitochondrial transfer say it is akin to creating designer babies, replacing faulty mitochondria with healthy ones would not be genetic engineering in the usual understanding of the term. It would not make a child smarter, sportier, more attractive, or otherwise different from what his genome and environment would produce in the normal way.
Britain said the proposed new rules would be subject to public scrutiny and parliament's approval.
Many scientists, campaigners and medical experts welcomed the government's decision.
Jeremy Farrar, director of the Wellcome Trust international medical charity, urged the government to "move swiftly so that parliament could debate the regulations at the earliest opportunity and families affected by these devastating disorders can begin to benefit".
Peter Braude, a professor of obstetrics and gynaecology at King's College London, welcomed the move, saying: "It is true that genetic alteration of disease risk is an important step for society and should not taken lightly."
"However the proposed changes to the regulations ensure it will be limited to informed couples, who understand from sad personal experience the significant effects of their disease, and are best placed to balance the risks of the technology with the possibility of having children without mitochondrial disease," he added.
Scientists are researching several three-parent IVF techniques.
One being developed at Britain's Newcastle University, known as pronuclear transfer, swaps DNA between two fertilised human eggs. Another, called maternal spindle transfer, swaps material between the mother's egg and a donor egg before fertilisation.
A British ethics panel review of the potential treatments in 2012 decided they were ethical and should go ahead as long as research shows they are likely to be safe and effective.
Because Britain is in the vanguard of this research, ethical concerns, political decisions and scientific advances are closely watched around the world.
Britain's public consultation on the draft regulations began on Thursday and was scheduled to run until May 21, 2014. (Editing by Alison Williams)