DRAFT rules have been announced that would make Britain the first country in the world to offer "three-parent" fertility treatments to families who want to avoid passing on incurable diseases to their children.

The regulations published by the UK Government's Department of Health could come into force by the end of this year, following a period of public consultation.

Ministers have agreed to amend fertility law in order to prevent a range of inherited diseases caused by faulty mitochondria, tiny energy-generating powerhouses in cells that have their own DNA.

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Under the new rules, which would apply to Scotland, IVF (In-Vitro Fertilisation) clinics would be able to replace a baby's defective mitochondrial DNA with healthy DNA from a female donor's egg.

Controversially, it would result in babies having DNA from two parents plus a tiny amount of extra DNA from a second "mother".

While many doctors and scientists applaud the move, pointing out it could eliminate terrible diseases, critics argue "mitochondrial transfer" could be a slippery slope leading to designer babies.

Dr David King, director of pressure group Human Genetics Alert, said the techniques were "unethical" and had not passed the necessary safety tests.

But Liz Curtis, from the Lily Foundation, which funds research into mitochondrial diseases, said: "These IVF techniques will eradicate mitochondrial disease for some families, offering the opportunity to have a healthy child. We hope the approval will not take too long."

The Human Fertilisation and Embryology Authority will decide if treatment can go ahead, on a case-by-case basis.