The aim is to get healthy offspring free of inherited genetic disorders

BBC News explains how and why.

Why are the UK researchers doing it?

The groundbreaking work aims to prevent mothers from passing certain genetic diseases on to their unborn babies.

The researchers are focusing on a group of conditions called mitochondrial myopathy, which include types of muscular dystrophy.

These cause muscle weakness and wasting, making it difficult to move normally - some may need to use a wheelchair.

At present no treatment for these exists, although things can be done to help with the symptoms.

Is there anything unique about these diseases?

Yes. Most genetic diseases result from faults in the DNA of the organ at the heart of the cell, the nucleus.

However, diseases of this type are caused by faulty genetic material contained in different cell structures called mitochondria.

Mitochondria are complex structures, which float in the cytoplasm outside of the nuclei of most cells.

They are responsible for energy production inside the cell - and can be compared to minute batteries.

It is thought that millions of years ago, they were thought to be free-living bacteria, but it is now known they cannot exist outside of our cells.

But, because of their unusual heritage, they have their own DNA - which is inherited from the mother only.

It is only a tiny proportion of the total DNA of the cell - 37 genes compared with the 25,000 genes found in the nucleus.

But if these few genes are faulty, the consequences can be devastating.

What do scientists want to do?

They plan to experiment on a fertilised egg from a woman who carries faulty mitochondrial DNA.

The egg, discarded from IVF treatment, will be at such an early stage of its development that it will still only be formed of one cell.

The scientists will remove tiny structures called pronuclei, which are destined to become the egg's nucleus.

These will then be put into a fertilised egg from another women with only healthy mitochondrial DNA.

Although this second egg has been fertilised, the pronuclei DNA from the male and female that made this egg will be removed.

The result would be an embryo with pronuclei DNA from the parental egg and sperm but mitochondria - and mitochondrial DNA - from the donor egg.

This embryo would share the over-whelming majority of its DNA - and almost all its physical characteristics - with its parents.

But scientists believe it would also be free of the risk of mitochondrial disease as it would have different mitochondria from its mother.

Why is this controversial?

Opponents believe it is wrong to experiment with material that has the potential to become a human life.

There are also doubts about the safety of the technique, with concerns about high rates of abnormalities.

The researchers say they have no intention of allowing the embryo to develop into a fully formed baby.

Was permission straightforward?

No. Current legislation covering reproductive technology makes no explicit provision for such ground-breaking science, which was not envisaged at the time the original law was drawn up.

The Human Fertilisation and Embryology Authority initially rejected an application to push ahead with the research, ruling that it was not permitted by the Human Fertilisation and Embryology Act 1990.

However, following an appeal it has now approved the application.