Eight more British families are to sidestep the law by following the example of a couple who travelled to America to use selective embryo screening to create a so-called "designer baby" to help save the life of their first child.

The case has led to calls for changes in the law to allow the procedure to be carried out in this country.

Michelle and Jayson Whitaker want to use stem cells from newborn son Jamie's umbilical cord to treat his brother who has a life-threatening blood disorder. Jamie's selection was carried out during the embryo stage, when a near-perfect match to his four-year-old brother, Charlie, was made.

The UK doctor helping the Whitakers, Mohammed Taranissi, said yesterday he has been contacted by 12 other couples seeking similar help.

He said that because three or four of the couples had long-standing fertility problems only eight of them had a realistic chance of reaching the embryo selection stage.

Evan Harris, the Liberal Democrat health spokesman and the Whitakers' MP until their recent move to Derbyshire, said the Government should allow Parliament to re-examine the law on fertility. "It is high time the Government allowed proper public and parliamentary debate and amendment to the Human Fertility and Embryology Act to permit this sort of treatment," he said. "The Act is 13 years old and is no longer up- with clinical developments."

Jamie Whitaker is not the first British-born baby selected to help cure a sibling: a couple whose child was suffering from leukaemia and needed a bone marrow transplant took the same route in 2001.

Other babies "designed" to help siblings have been born in the US. Jamie's brother suffers from a rare form of anaemia that requires a regular, painful treatment. It can only be cured by a transplant of stem cells from a sibling with a perfect tissue match.

Yesterday, the Human Fertilisation and Embryology Authority (HFEA) defended its decision not to allow treatment. A spokeswoman said: "We have to look at the benefit for the embryo, not just the sibling. Perhaps some day in the future our policy will change. But at the moment we have to be quite strict in the way we issue licences, on a case-by-case basis, and looking at the scientific, medical, and moral issues.

"There is clear guidance. HFEA policy states that women are allowed to have treatment only for the benefit of the embryo."

Stem cells have been collected from Jamie and tests will be done to see whether he has the same condition as his brother.

The Human Fertilisation and Embryology Authority said it was acceptable to test and select embryos to prevent the birth of a baby with a genetic disease, but not to select them for the purpose of helping another child.