Britain has become the first country in the world to allow the creation of human embryos from the DNA of three people, a technique intended to help mothers avoid passing on genetically degenerative diseases to their babies.
The bill granting the controversial techniques was passed on Tuesday by the House of Lords, after being approved earlier this month by the House of Commons.
Experts say only about a dozen women in the UK will likely be eligible for treatment every year.
The controversial techniques involve altering an egg or embryo before it is transferred into a woman which had previously been forbidden by British law.
They are intended to avoid passing on defects in the mother's mitochondria, which can result in diseases including muscular dystrophy, heart, kidney, liver failure and severe muscle weakness.
Without pain and suffering
In an open letter ahead of a parliamentary vote, charities and advocacy groups said the opportunity "offers families the first glimmer of hope that they might be able to have a baby that will live without pain and suffering".
In their letter, groups including the US-based United Mitochondrial Disease Foundation, the Australian Mitochondrial Disease Foundation and groups from France, Germany, Britain and Spain, described mitochondrial disease as "unimaginably cruel".
"It strips our children of the skills they have learned, inflicts pain that cannot be managed and tires their organs one by one until their little bodies cannot go on any more," they wrote.
They said they were aware "that no novel medical procedure is without risk", but had "absolute confidence" in scientific panels that have examined the technique.
However, the treatment is feared by critics who say it effectively allows "designer babies" because it would involve implanting genetically modified embryos into women.
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Image: Woman undergoing fertility treatment from Shutterstock