Molly Nash with her brother Adam

Mrs Nash underwent four IVF cycles before becoming pregnant with Adam and each time embryos were screened to eliminate those carrying the Fanconi anaemia gene.

The resulting boy, Adam, born in August, has now been able to donate stem cells collected from his umbilical cord to his older sister to replace her failing bone marrow.

The procedure took place late last month at Fairview-University Medical Center in Minneapolis and doctors there believe Molly now has an 85-90% chance of recovery.

Children with Fanconi anaemia suffer from severe bleeding and immune system disorders and invariably die by the time they reach eight or nine.

The only effective treatment is a bone marrow transplant from a perfectly matched healthy sibling.

Not allowed under UK law

The Head of ethics and policy for the British Medical Association, Dr Vivienne Nathanson, suggested the technique would not be allowed under British law because of the possibility the child was being seen simply as a "medical product".

We would have very serious concerns that he is a commodity rather than a person

Dr Vivienne Nathanson

"You obviously have sympathy with the family but we have to have concern about the second child. We would have very serious concerns that he is a commodity rather than a person," she told BBC News Online.

In the UK, PGD has been used in five clinics for the past 10 years. The technique has resulted in the birth of about 20 healthy babies, who would otherwise have been at risk of serious genetic diseases such as cystic fibrosis or haemophilia.

James Yeandel, spokesperson for the regulatory body the Human Fertilisation and Embryology Authority, said that an application for use of PGD for a purpose such as creating a transplant match would have to be considered by the authority's licensing committee.

But he added: "Use of PGD has been approved for a number of serious genetic diseases on a named disorder basis. It can not be used for any social, physical or psychological reasons."

And the selection technique has been vigorously condemned by pro-life organisation Life.

Life spokesman Kevin Male added: "Adam was the fifteenth embryo created which meant fourteen people were killed before him. In essence a white coated technician brought this human being into the world simply as a means to an end."