The new law means IVF clinics will be able to offer a procedure in which donor DNA from a 'second mother' is implanted into an egg to help prevent hereditary diseases being passed from mother to child. The technique could protect babies from inheriting mitochondrial diseases caused by DNA mutations in a cell's energy source (the mitochondria). Currently one in every 200 babies born in the UK has a severe mitochondrial disease, which can cause muscle wastage, nerve damage, loss of sight and heart failure.
The procedure, which was developed by British scientists, involves removing the nucleus from an affected woman's egg and transferring it to a donor egg that has healthy mitochondria DNA and has been fertilised by the father's sperm, in a lab.
The egg containing the DNA from three adults would then be implanted into the mother and the pregnancy could progress normally. The fraction of DNA from the donor is minuscule and would not influence the child's physical appearance or personality.
Once the new rules are brought in, the fertility regulator, the Human Fertilisation and Embryology Authority (HFEA), will decide whether a treatment can go ahead on a case-by-case basis. Mitochondrial transfer will only be allowed when there is a "significant risk" of disability or serious illness.
Children born after mitochondrial transfer will not be entitled to discover the identity of the "third parent" donor.
While many doctors and scientists applaud the move, critics argue that it could open the door to the possibility of creating 'designer' babies, whose eye colour, gender and intelligence could be selected by parents.
There is also concern surrounding the fact that this procedure would be the first to introduce genetic changes that will be passed on not only to the intended child, but to any children that individual goes on to have in the future.