MPs will hold a crucial vote later to decide whether to allow the creation of babies using DNA from three people.
They will have a free vote on the technique, which is aimed at preventing deadly genetic diseases being passed from mother to child.
The UK could become the first country to introduce laws to allow the creation of babies from three people.
It has sparked fierce ethical debate and senior Church figures have called for the procedure to be blocked.
However, if there is a “yes” vote in the Commons, then the first three-person baby could be born as soon as next year. It could eventually help about 150 couples a year.
The technique, which was developed in Newcastle, should help women like Sharon Bernardi, from Sunderland, who lost all seven of her children to mitochondrial disease.
Mitochondria are the tiny compartments inside nearly every cell of the body that convert food into useable energy. They have their own DNA which does not affect characteristics such as appearance.
Defective mitochondria, which are passed down only from the mother, lead to brain damage, muscle wasting, heart failure and blindness.
The technique uses a modified version of IVF to combine the DNA of the two parents with the healthy mitochondria of a donor woman.
It results in babies with 0.1% of their DNA from the second woman and is a permanent change that would be passed down through the generations.
The government backs the measure in principle.
However, MPs will be given a free vote, as it is an issue of conscience, rather than being forced down party lines.
Prof Doug Turnbull, the director of the Wellcome Trust centre for mitochondrial research where the technique was pioneered, urged MPs to vote in favour.
He told the BBC News website: “This is research that has been suggested by the patients, supported by patients and is for the patients, and that’s an important message.”
That message was backed up by calls from British Nobel Prize-winning scientists and 40 leading scientists from 14 countries.
But not everyone agrees.
What will happen?
- There will be a 90-minute debate in the House of Commons, expected after 14:00 GMT
- This will be followed by the vote
- The House of Lords will also have to vote, although those close to the issue believe the Commons’ vote will be the crucial one
- If the procedure is given the go-ahead, the UK fertility regulator will be able to license fertility clinics
Last week 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.
David King, from the campaign group, said: “Once you cross the ethical line, it is very hard not to take the next step of designer babies.
“All our experience says we are pushed down slopes by thousands of people who are paid to make sure we go that way.”
Dr Gillian Lockwood, a reproductive ethicist, told the BBC it was a “small change” in the legislation.
“The biggest problem is that this has been described as three-parent IVF. In fact it is 2.001-parent IVF,” she said.
“Less than a tenth of one per cent of the genome is actually going to be affected. It is not part of what makes us genetically who we are.
“It doesn’t affect height, eye colour, intelligence, musicality. It simply allows the batteries to work properly.”
A review by the Nuffield Council on Bioethics, as well as a public consultation by the fertility regulator, argued the creation of three-person babies was ethical.
Three scientific reviews by the Human Fertilisation and Embryology Authority (HFEA) suggest the technique is “not unsafe”.
Yet some scientists argue those reviews were flawed.
Dr Ted Morrow, from the University of Sussex, believes there are still uncertainties.
“I have some concerns about the safety, I’m really not happy that the reviews have been as exemplary as other people think they are.”
Prof Lisa Jardine, former chair of the HFEA, said the safety issue was a “red herring”.
Rachel Kean, whose aunt died from mitochondrial disease, told BBC Breakfast that a yes vote would “prevent some of the cruellest and most devastating diseases, not just for the next generation but generations after”.
She said there had been an “unprecedented” amount of scrutiny into the regulation and a lot of misinformation about “designer babies”.
If politicians vote it through, then the HFEA is expected to give Newcastle a licence to carry out the procedure.
The first attempt could take place this year, which could lead to the first birth in 2016.
The religious communities concerns are fair on principle, though it is debatable how many of those that opposes the procedure actually understands it.
Much opposition is with regard to the fact that any child produced from this procedure would effectively have three genetic parents and this is deemed to be “un-godly”; yet as the BBC have tried to explain in their schematics, a child born of this procedure would only have the chromosomal DNA from their 2 biological parents and the mitochondrial DNA from their “donor parent”.
As it stands, every person on this earth has pretty much the same mitochondrial DNA as their mother only as this is passed on from the mother’s egg and not the fathers sperm.
It is an issue with the mother’s mitochondria that causes the metabolic diseases of affected children. Such a procedure would save many lives and stop the suffering of children and their families without doing anything to make those children genetically any less of their parents on a chromosomal level.
Anyone who would claim that such diseases are gods will can never have their minds changed until they are affected by such things.
The question that this issue leads to is: why does a religious organisation such as the Anglican Church of England have any say in the way that laws are made in a secular country?