UK scientists have been given the go-ahead by the fertility regulator to genetically modify human embryos.

It is the first time a country has considered the DNA-altering technique in embryos and approved it.

The research will take place at the Francis Crick Institute in London and aims to provide a deeper understanding of the earliest moments of human life.

It will be illegal for the scientists to implant the modified embryos into a woman. But the field is attracting controversy over concerns it is opening the door to designer - or GM - babies. DNA is the blueprint of life - the instructions for building the human body. Gene editing allows the precise manipulation ofDNA.

In a world-first last year, scientists in China announced they had carried out gene editing in human embryos to correct a gene that causes a blood disorder.

Prof Robin Lovell-Badge, a scientific advisor to the UK's fertility regulator, told the BBC: "China has guidelines, but it is often unclear exactly what they are until you've done it and stepped over an unclearboundary.

"This is the first time it has gone through a properly regulatory system and been approved."

"The experiments will take place in the first seven days after fertilisation. During this time we go from a fertilised egg to a structure called a blastocyst, containing200-300 cells."

The work will be led by Dr Kathy N iakan, who has spent a decade research -inghuman development

Earlier this year, she explained why she had applied to edit human embryos:

"We would really like to understand the genes needed for a human embryo to develop successfully into a healthy baby.

"The reason why it is so important is because miscarriages and infertility are extremely common, but they're not very well understood."

Out of every 100 fertilised eggs, fewer than 50 reach the early blastocyst stage, 25 implant into the womb and only 13 develop beyond three months.

And at the blastocyst stage, some cells have been organised to perform specific roles - some go on to fonn the placenta, others theyolksacand others ultimately us.

How and why this takes place is unknown - but some parts of our DNA are highly active at this stage. It is likely these genes are guiding our early development, but it is unclear exactly what they are doing or what goes wrong in miscarriage.

The researchers will alter these genes in donated embryos. The regulator, the Human Fertilisation and Embryology Authority (HFEA), yesterday gave its approval and the experiments could start in the next few months.