Britain has become the first country in the world to give the go-ahead for licensed "three parent" baby treatments for inherited diseases.
The decision, described as "historic" and "momentous", was taken unanimously at a board meeting of the UK fertility regulator, the Human Fertilisation and Embryology Authority (HFEA).
It means IVF clinics are now free to apply for permission to carry out mitochondrial replacement therapy (MRT), which aims to prevent babies and future generations inheriting devastating genetic disorders.
Children conceived using the technique will receive a tiny amount of DNA from a third person besides their mother and father, an egg donor.
Scientists at the University of Newcastle, which has pioneered the treatments, hope to treat 25 women a year with NHS funding.
The first patients could be given the therapy next spring, and the first babies with three genetic parents born nine months later.
The HFEA stressed it had taken a careful approach which meant only the most serious cases would be treated. In total, up to 3,000 women are likely to qualify for the therapy.
Chairwoman Sally Cheshire said: "Today's historic decision means that parents at very high risk of having a child with life-threatening mitochondrial disease may soon have the chance of a healthy, genetically related child. This is life-changing for those families."
The move came after an independent panel of experts cleared away remaining safety hurdles to recommend "cautious adoption" of MRT.
Last year, the UK became the first country in the world to make mitochondrial replacement officially legal after MPs and peers voted in favour of allowing it.
But the treatments could not go ahead without a green light from the HFEA.
Final approval of individual treatments will only come after the regulator has considered each application on a patient-by-patient basis.
Professor Adam Balen, chairman of the British Fertility Society that represents IVF clinics, said: "This marks a momentous and historic step and we hope families next year will begin their journey to eradicate these genetic diseases."
Fertility doctors carrying out the treatments will aim to replace abnormal genes in the mitochondria, rod-like power plants in cells that generate energy.
Mitochondria only hold around 0.1% of a person's DNA, which is always inherited from the mother and has no influence over individual characteristics such as appearance and personality.
It is quite separate from the DNA in the cell nucleus which house the vast majority of an individual's genes.
But when mitochondrial DNA (mtDNA) goes wrong, the results can be catastrophic, leading to a wide range of potentially fatal conditions affecting vital organs, muscles, vision, growth and mental ability.
MRT, conducted by fertility doctors, involves removing faulty mitochondria and substituting healthy versions from a donor.
It is carried out by transferring the genetic material that effectively encodes a baby's identity to a donor egg whose own nuclear DNA has been removed.
Two different techniques may be employed, either before or after fertilisation. The end result is the same - an embryo containing healthy mitochondria from the donor and nuclear DNA from the baby's mother and father.
In theory, mitochondrial replacement can not only prevent a child developing inherited diseases, but also protect future generations.
Despite the "cautious" approach adopted by the HFEA, some critics have argued that the regulator has acted too fast.
They point out that unforeseen problems may occur once the procedure is used to create human babies. For instance, replacing mtDNA might have more of an impact on personal traits than has been envisaged.
There is also said to be a real danger of small amounts of defective mtDNA slipping through and ending up in the children.
All the women to be initially treated will be closely monitored, with their outcomes written up in scientific journals.
HFEA chief executive Peter Thompson dismissed any suggestion that they were being used as "human guinea pigs".
He said: "I would whole-heartedly reject any such label that these women are guinea pigs. The key thing here is consent.
"These are women who have experienced terrible suffering that has affected them and their families. If they wish to try these new treatments that must be a decision for them."
The leader of the expert panel that advised the HFEA said there was no question of the techniques being used as part of regular infertility treatment for older women with ageing mitochondria.
Dr Andrew Greenfield said: "Some people think that, particularly eggs from older women, have a mitochondrial functional impairment that is responsible for the difficulties they have in establishing pregnancy.
"We make it absolutely clear here that we don't support the use of this technique for this practise."
Ms Cheshire said the decision marked the first time regulated use of mitochondrial replacement treatments had been allowed in the world.
Professor Sir Doug Turnbull, director of the Wellcome Centre for Mitochondrial Research at the University of Newcastle, said: "We are delighted by today's decision as it paves the way (toward) offering mitochondrial donation as part of an NHS-funded package of care for families affected by mitochondrial DNA disease.
"Newcastle is a major referral centre for the women with mitochondrial DNA mutations in the UK and it will be hugely welcomed as it provides them (with) reproductive choice.
"In Newcastle, we will be aiming to treat up to 25 carefully selected patients a year with the mitochondrial donation technique as a clinical risk reduction treatment. We will also provide long term follow-up of any children born."
Newcastle Fertility Centre is looking for healthy women up to the age of 35 who might consider donating eggs for MRT treatments.
Because of the need for close monitoring, they will have to live in the north-east region.
The team, based at Newcastle upon Tyne Hospitals NHS Foundation Trust and the University of Newcastle, said it would be applying for the first MRT licence "within 24 hours".
Professor Mary Herbert, from Newcastle Fertility Centre, said: "It is enormously gratifying that our many years of research in this area can finally be applied to help families affected by these devastating diseases."
Robert Meadowcroft, chief executive of the charity Muscular Dystrophy UK which supports sufferers of mitochondrial diseases, said: "Today, this historic decision will open the door to the first licensed treatments being offered to eligible women affected by mitochondrial disease.
"Families have, understandably, had to wait through years of thorough ethical, safety and public reviews.
"We know of many women who have faced heartache and tragedy, and the sorrow of stillbirths, while trying to start their own family, and this decision gives them new hope and choice for the first time.
"We recognise this approach is not without some uncertainty, and, in any trial, success cannot be guaranteed. However, it is important that women are able to make informed choices by understanding the risks and the potential benefits."
Dr David King, from the watchdog group Human Genetics Alert, said the HFEA had approved "experiments on babies" using technology that was "dangerous and medically unnecessary".
He accused experts backing the treatments of "shameless emotional blackmail and scientific misrepresentations".
Dr King added: "This decision opens the door to the world of GM (genetically modified) designer babies."
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