Churches oppose three-person baby plan
Senior church figures have called on the UK government to block the creation of babies from three people.
The Church of England and the Catholic Church in England and Wales said it was not clear the technique - adding a donor woman's mitochondria to another woman's egg - was safe or ethical.
But a group of scientists has urged MPs to approve the procedure - intended to stop deadly mitochondrial diseases.
Ministers want to allow the technique and MPs will debate it on Tuesday.
Mitochondria are tiny compartments found within cells within the body, and their most crucial role is to convert energy locked in food into energy the cell can use.
About one in every 6,500 babies is born with mitochondrial disease, which can be fatal.
Mitochondria are passed to a child from the mother, and the proposed technique involves adding healthy mitochondria from a donor woman to an egg from another, then fertilising it with one man's sperm.
The Reverend Brendan McCarthy, Church of England adviser on medical ethics, said: "We need to be absolutely clear that the techniques that will be used will be safe, and we need to be absolutely sure that they will work."
He also said the ethics of the issue should be properly discussed before a decision was made, adding: "What's the rush?"
'Not playing God'
Sharon Bernardi, who lost all of her seven children to mitochondrial disease, said the proposed technique was not about changing the colour of a child's eyes or hair.
"This is trying to make children survive," she said.
"We're not playing God or anything."
She said her first baby died within 28 hours of birth, and she lost five other children while they were babies - and her son Edward at the age of 21.
"I would ask them [the Church of England] desperately to please look at the bigger picture and look at the children who have been affected by this dreadful disease," she said.
"No child should be born with a disease that's going to cut their life short.
"I can't believe anybody from the Church would want that."
The Right Reverend John Sherrington, a Roman Catholic bishop, said many people were "rightly concerned" about the proposal.
"No other country has allowed this procedure and the international scientific community is not convinced that the procedure is safe and effective," he said.
"There are also serious ethical objections to this procedure, which involves the destruction of human embryos as part of the process."
But in a letter to the Guardian, 40 scientists from 14 countries said the technique offered "some affected families the opportunity to have healthy children".
They said the UK had run an "exemplary and internationally admired process" to consider the issue since 2007, and they called on Parliament to approve the proposed change.
"The UK hosts a world class team at Newcastle University developing this technology, which is ideally placed to be among the first to treat patients," they added.
The House of Commons debate on Tuesday is on a proposed alteration to the the Human Fertilisation and Embryology Act 1990, to "enable mitochondrial donation".
By James Gallagher, health editor, BBC News website
This idea has raised ethical questions since it was first proposed 15 years ago.
Doctors say it could prevent repeats of Sharon Bernardi's experiences.
But critics say it introduces a permanent change to DNA that would be passed through the generations, raises concerns about the use of embryos and opens the door to designer babies.
Those are the issues MPs will be debating on Tuesday.
Three reviews by the fertility regulator suggest the technique is safe, but nobody can be certain until it is finally tried in people.
When I spoke to the head of the mitochondrial research centre in Newcastle, he said he was "anxious" ahead of the vote.
But if the technique is approved, then scientists expect the first attempt before the end of the year.
How is it done?
Two separate techniques are being devised, but they both share the same common principle - take the DNA from the parents and combine it with healthy mitochondria from a female donor.
Asked about the Church of England's comments - first published in the Daily Telegraph - Prof Jeremy Farrar, director of the Wellcome Trust, said the Church had a "right" to interject.
But he said he was surprised it had done so "at this late stage".
He said there had been a series of scientific, ethical and public consultations, adding: "This process has been going on for seven years or more."
Prof Farrar said scientists cannot be sure the technique is "100% safe" until it is used on humans, but that jump always had to be made with new techniques.
"I don't think there's been any more rigorous look at any scientific endeavour coming into humans," he added.