Draft rules allowing the introduction of new treatments that could see the creation of babies with three genetic parents have been announced by the Government.
The regulations could come into force by the end of this year following a period of public consultation.
Ministers have agreed to amend fertility law in order to prevent a range of inherited diseases caused by faulty mitochondria, tiny energy-generating powerhouses in cells that have their own DNA.
Under the new rules, IVF (In-Vitro Fertilisation) clinics will be able to replace a baby's defective mitochondrial DNA with healthy DNA from a female donor's egg.
Controversially, it would result in babies having DNA from two parents plus a tiny amount of extra DNA from a second "mother".
While many doctors and scientists applaud the move, pointing out that it could eliminate terrible diseases, critics argue "mitochondrial transfer" could be a slippery slope leading to designer babies.
Dr David King, director of the pressure group Human Genetics Alert, said: "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.
"The techniques have not passed the necessary safety tests so it is unnecessary and premature to rush ahead with legalisation.
"The techniques are unethical according to basic medical ethics, since their only advantage over standard and safe egg donation is that the mother is genetically related to her child. This cannot justify the unknown risks to the child or the social consequences of allowing human genome modification."
But Dr King's was a lone voice among those of leading experts who lined up to welcome the move.
Professor Peter Braude, head of obstetrics and gynaecology at King's College London, said: " I am pleased that the Government has been brave enough to follow through on their promises given during the 2008 revision of the Human Fertilisation and Embryology Act, to bring before Parliament an option to help a small but deserving portion of society blighted with the spectre of transmitting mitochondrial disease to their children.
"Although rare, the effects of mitochondrial disease are devastating on those families, and the technology proposed will bring hope to those carrying the disorders.
"It is true that genetic alteration of disease risk is an important step for society and should not be 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."
Dr Jeremy Farrar, director of the Wellcome Trust, Britain's biggest research charity, said: " It is now almost a year since a major public consultation found broad support for the use of new IVF techniques for preventing mitochondrial diseases, so we are pleased that the Government has now published draft regulations that would permit this.
"Once further public consultation on the detail of these regulations is complete, we urge the Government to move swiftly so that Parliament can debate the regulations at the earliest opportunity and families affected by these devastating disorders can begin to benefit."
Doug Turnbull, Professor of Neurology at the University of Newcastle, said: " I am delighted that the Government has published the draft regulations. This is very good news for patients with mitochondrial DNA disease and an important step in the prevention of transmission of serious mitochondrial disease"
Around one in every 6,500 babies born in the UK has a severe mitochondrial disease. Although rare, the disorders can be passed to future generations through the maternal line.
Examples of mitochondrial diseases include conditions that cause muscle wasting, nerve damage, loss of sight and heart failure.
Robert Meadowcroft, chief executive of the Muscular Dystrophy Campaign, said: " News that the wait for proposed amendments to genetic research regulations to be shared with the public is over will be welcomed by many families living with mitochondrial disease.
"We have supported the Government's review of the mitochondrial transfer IVF technique throughout, in the firm belief that open, thorough and transparent dialogue is critical. However, it will soon be two years since the initial consultation with the public was announced and three since the review began.
"There have been lengthy waits at every stage, and we now call on the Government to ensure that regulations are passed before the next general election, so that the technique can be moved towards clinical trials as soon as possible."
He added: "E ncouragingly, we have seen that, when given in-depth information, the majority of people in the UK are broadly supportive of this technology. We now need to see a prompt, efficient discussion with the public on the rules that will govern how it is taken forward."
The new consultation is not to debate whether mitochondrial transfer should be allowed, but how it should be implemented.
Once the rules are brought in, it will be up to the fertility regulator, the Human Fertilisation and Embryology Authority (HFEA), to decide whether a treatment can go ahead on a case-by-case basis.
Mitochondrial transfer will only be allowed when there is a "significant risk" of disability or serious illness.
Children born after mitochondrial transfer will not be entitled to discover the identity of the "third parent" donor.
Liz Curtis, from the Lily Foundation, which funds research into mitochondrial diseases, said: "The publication today of the draft regulations is welcomed by The Lily Foundation. We meet too many families on a daily basis whose worlds have been turned upside down by the devastating effects of mitochondrial diseases.
"These IVF techniques will eradicate mitochondrial disease for some families, offering the opportunity to have a healthy child. We hope the approval will not take too long, so these families can benefit from this as soon as possible and hopefully see a little light at the end of a dark tunnel."