Britain could become the first country in the world to permit babies to be born with three genetic parents by the end of next year.
A landmark decision by the Department of Health opens the door to controversial treatments for inherited diseases that make use of donated DNA from a second donor "mother".
New regulations to fertility law allowing the procedures will be issued for public consultation later this year and then debated in Parliament. If MPs find them ethically acceptable the first patients could be treated within months. It is envisaged that between five and 10 "three-parent" babies would be born each year.
Allowing the currently illegal techniques would mark a turning point because it means, for the first time ever, altering the "germ line" made up of inherited DNA. Experts point out that only the tiny amount of DNA in a cell's "battery packs", the mitochondria, would be changed. DNA in the nucleus, which determines individual characteristics such as facial features and eye colour, would remain intact. But some critics believe the move would mark a slippery slope leading to "designer babies" and eugenics.
The aim of the in-vitro fertilisation (IVF) treatments is to stamp out serious mitochondrial diseases which can be passed from a mother to her children. Around one in 200 babies are born each year in the UK with defects in the mitochondria, rod-like bodies that supply cells with energy. One in 6,500 is seriously affected and can suffer potentially life-threatening diseases including a form of muscular dystrophy and conditions leading to hearing and vision loss, heart, lung and liver problems, and bowel disorders. An estimated 12,000 people in the UK live with the diseases. The new techniques result in defective mitochondrial DNA (mDNA) being replaced by a healthy version supplied by a female donor.
Speaking to journalists in London, she said she personally felt "very comfortable" about altering mitochondrial DNA, even though it was part of the germ line. "I do think quite carefully about ethics, I always did as a clinician and I still do, perhaps because my father was a theologian," she said. "I am comfortable with this. I think we will save some five to 10 babies from being born with ghastly disease and early death without changing what they look like, or how they behave, and it will help mothers to have their own babies."
Under current law, such procedures are banned because any tampering with inherited genetic material is illegal. However a safeguard has deliberately been left in the Human Fertilisation and Embryology Act allowing this blanket rule to be changed by Parliament in exceptional circumstances. Dame Sally said: "There are clearly some sensitive issues here, but ... it's clear there is general support to allow these treatments subject to strict safeguards. So what we're going to do is move forward." She made it clear that the outright ban on altering nuclear DNA would remain in place and there was no likelihood of that position changing "in the foreseeable future".