File photo: In a leap forward for gene editing, it offers hope for children with cystic fibrosis and sickle cell anaemia, which can also be caused by one error in DNA. Picture: Pixabay

London - Scientists have taken another step towards designer babies by removing a disease from a human embryo.

In a world first, Chinese researchers corrected a single error in the three billion "letters" of our genetic code.

They used a targeted chemical reaction to rewrite human DNA, removing the mutation which causes a blood disorder called beta-thalassemia.

In a leap forward for gene editing, it offers hope for children with cystic fibrosis and sickle cell anaemia, which can also be caused by one error in DNA.

However, the success, in a field now being led by China, will be controversial among critics of "designer babies" who fear it could potentially be used to select their eye colour to order.

Dr Junjiu Huang, from Sun Yat-sen University in China, who led the study, told the BBC: "We are the first to demonstrate the feasibility of curing genetic disease in human embryos by base editor system."

Darren Griffin, professor of genetics at the University of Kent, said: "For many years, we have been saying that direct gene editing in embryos is some way in to the future. Now the future is here and there is much to consider."

Currently parents in danger of passing killer genetic diseases to their children can only try to select an embryo which does not carry the mutation. The latest study suggests a disease could be removed from the embryo during IVF.

However, experts say the breakthrough is still a long way from helping children and the embryos used in the study were kept alive for only a few days. They were created using cloned cells taken from a patient with beta-thalassemia.

Professor Shirley Hodgson, of St George’s University of London, hailed the "novel gene editing technique". But she stressed it could only correct disease mutations caused by a single base change, which is the case for only a small minority of genetic disorders.