So-called "three-parent babies" have today been approved by the UK's fertility regulator.
The controversial move involves the use of two women's eggs and one man's sperm in a ground-breaking procedure that hopes to eradicate some genetic diseases.
The first child born as a result of this procedure could be born by the end of 2017 after the "decision of historic importance" was granted on Thursday by the Human Fertilisation and Embryology Authority (HFEA).
Chair Sally Cheshire said: "This is about cautious go ahead, not gung-ho go ahead and there is a long way to go.
"I'm sure patients will be really pleased by what we've decided today."
But critics have warned this opens up the way for 'designer' babies.
The procedure hopes to eradicate the incurable mitochondrial disease that leaves babies without enough energy to breathe. The illness is caused by faulty genes and is passed down through the mother's egg.
Doctors in Newcastle, who first developed the technology, are expected to be the first to offer the advanced version of IVF.
Two eggs, one from a donor and one from the mother, will be fertilised with the father's sperm. Scientists will then remove the nucleus from both embryos and insert the parents' nucleus into the donor embryo.
It is thought the procedure will help 25 couples a year to conceive a healthy child.
But some Christians have criticised the move.
The Roman Catholic Church opposed the change in law which allowed the process to be legalised, suggesting that it "dilutes parenthood." The Church of England, meanwhile, said it wasn't necessarily opposed to the procedure in principle but wanted more research on the impacts as well as further debate on the ethics of the issue.
Dr David King, from the campaign group Human Genetics Alert, said: "This decision opens the door to the world of genetically-modified designer babies.
"Already, bioethicists have started to argue that allowing mitochondrial replacement means that there is no logical basis for resisting GM babies, which is exactly how slippery slopes work."
But Professor John Bryant, a Christian biosciences expert at the University of Exeter, told Christian Today: "The idea that this puts us on a slippery slope to designer babies is nonsense.
"The technology for doing that involves altering nuclear genes - ie. the main set of genes - and that technology has been available for over 25 years.
"Nobody has suggested that it should be legal to use it with human embryos even though it is very widely used with other mammals, especially mice for use in medical research."
