The pregnancy is the result of an embryo screening technique that has never previously been applied in Britain, to prevent parents passing on the disease to their offspring.

If the couple had conceived naturally and the baby had inherited the altered version of the gene, called BRCA1, it would have had an 80 per cent chance of developing breast cancer and a 60per cent chance of ovarian cancer.

By screening embryos during in-vitro fertilisation, doctors could be sure the child would be free of the mutation.

Paul Serhal, of University College Hospital in London, who carried out the screening, said the technique offered hope for families who had suffered repeated cases of inherited breast cancer.

''All these couples have a very strong family history of breast cancer,'' he said, ''You have certain couples who have been plagued by this for generations.''

He refused to give any details about the mother who has requested to remain anonymous, but it is believed she is aged 27.

The screening technique used by Dr Serhal called pre-implantation genetic diagnosis involves creating embryos using IVF, then removing a cell from each of them when they have divided to give eight cells.

Genetic analysis of this cell can reveal whether each embryo contains the normal or mutated copy of BRCA1. The embryos with the mutated copy are discarded, while one or more embryos with the normal BRCA1 are implanted back into the mother's womb to begin the pregnancy.

Pre-implantation genetic diagnosis screening is now fairly routine for some severe inherited conditions such as cystic fibrosis and Huntington's disease, but applying it to conditions such as inherited breast cancer has proved controversial.

Because there is a chance that carriers of the mutation will not develop breast cancer, screening inevitably means some embryos that are screened out would have led healthy lives.

The disease is also not necessarily fatal and can be prevented by surgery to remove the breasts, before cancer develops.

The procedure had to be licensed by the Government's fertility watchdog, the Human Fertilisation and Embryology Authority.

Before granting the licence to Dr Serhal in 2007, it carried out a public consultation on whether screening in this case was morally acceptable.

''The outcome of this consultation paper was overwhelmingly positive,'' Dr Serhal said.

He has previously used pre-implantation genetic diagnosis to screen for a form of inherited bowel cancer and retinoblastoma, an inherited eye cancer.

The director of the centre for pre-implantation genetic diagnosis at Guy's hospital in London, Professor Peter Braude, said, ''Clearly, the decision as to whether PGD is appropriate for a couple will be made after a thorough discussion with knowledgeable genetic counsellors and clinical geneticists.'' Guardian