Peers are debating whether to make the historic move to allow the creation of babies with DNA from three people.
The modified form of IVF would be used to prevent babies inheriting deadly genetic diseases.
Questions over the legality and safety of the measure have been raised in the House of Lords.
If the House votes in favour, then the UK would be the first country to introduce laws to allow the creation of babies from three people.
Earlier this month, 382 MPs in the House of Commons voted in favour, and 128 against the technique that stops these genetic diseases being passed from mother to child.
Mitochondria are the tiny compartments inside nearly every cell of the body that convert food into useable energy.
But genetic defects in the mitochondria mean the body has insufficient energy to keep the heart beating or the brain functioning.
The structures are passed down only from the mother and have their own DNA, although it does not alter traits including appearance or personality.
The technique, developed in Newcastle, uses a modified version of IVF to combine the healthy mitochondria of a donor woman with DNA of the two parents.
It results in babies with 0.1% of their DNA from the second woman and is a permanent change that would echo down through the generations.
In the debate, health minister Lord Howe said there was an opportunity to offer “real hope” to families.
He stated the UK was leading the world and that three safety reviews by experts suggested it would be safe.
Lord Howe told the House: “Families can see that the technology is there to help them and are keen to take it up, they have noted the conclusions of the expert panel.
“It would be cruel and perverse in my opinion, to deny them that opportunity for any longer than absolutely necessary.”
Lord Deben, the former government minister John Gummer, countered that there were “real doubts about safety”.
He also voiced concerns about whether the creation of such babies would be legal.
“It is quite clear that there is considerable disagreement, let me put it simply like that, about whether this action is legal under European law.”
Baroness Scotland of Asthal, a former Labour attorney general, also questioned the legality asking: “Why the haste?
“Everyone agrees we have to get this right. If we’re going to do something which everyone agrees is novel, different and important internationally we really have to be confident that we are on solid ground. If we are not we give a disservice.”
Fertility doctor, Lord Winston, told the House there were comparison with the early days of IVF which was “also a set in the dark”.
He added: “I don’t believe my Lords, in spite of what we’ve heard this evening, that this technology threatens the fabric of society in the slightest bit.”
James Lawford Davies, a lawyer from Lawford Davies Denoon which specialises in the life sciences, told the BBC: “All of the legal arguments made in opposition to the regulations are hopeless.
“The regulations do not breach the Clinical Trials Directive which applies only to medicinal products.
“The regulations do not breach the EU Charter of Fundamental Rights and Freedoms which prohibit ‘eugenic practices’ as this is intended to prevent practices such as forced sterilisation and reproductive cloning, not treatments intended to prevent the transmission of disease.”
Speaking ahead of the vote, chief medical officer Prof Dame Sally Davies told the BBC: “These children die in the first few months or years of life, because their muscles collapse, their hearts fail, their brains don’t develop and they die. This is tragic.
“I would ask the Lords to vote for this as the Commons have and the mothers really want.”
The Catholic and Anglican Churches in England said the idea was not safe or ethical, not least because it involved the destruction of embryos.
Other groups, including Human Genetics Alert, say the move would open the door to further genetic modification of children in the future – so-called designer babies, genetically modified for beauty, intelligence or to be free of disease.
Estimates suggest 150 couples would be suitable to have babies through the technique each year.
If the measure goes ahead, the first “three-person” baby could be born next year.
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