The controversy over "saviour siblings", children created to help treat a brother or sister with a genetic disorder, was finally put to rest yesterday when the UK's highest court ruled it to be lawful.

Five law lords unanimously ruled that the law that set up the Human Embryology and Fertilisation Authority (HFEA) gave the authority power to license fertility treatment aimed at saving a sibling's life.

The test case centred on an appeal court judgment in April 2003 which overturned a high-court ban on using the treatment to help cure Zain Hashmi, now six, who was born with the potentially fatal disorder beta thamassaemia major.

His parents, Raj and Shahana Hashmi, from Leeds, went ahead with the treatment, hoping to have a baby with the same tissue type as their son so as to treat his rare blood disorder. The couple have so far had six unsuccessful attempts at IVF treatment, either failing to produce an embryo of the right tissue type or having a pregnancy end in miscarriage.

The case was taken to the lords by Josephine Quintavalle and her campaign group, Comment on Reproductive Ethics, who argued that the HFEA had no power to license the creation of a baby to save the life of another.

The authority did grant a licence allowing the Hashmis' treatment to go ahead, but this was challenged by Ms Quintavalle, who originally was successful when a high-court judge ruled that the authority had no power to grant the licence.

Mrs Hashmi, 38, and her husband had to fight a long legal battle for the treatment, which they believe is the only hope for their son. His body does not produce sufficient red blood cells; he takes a cocktail of drugs daily and needs regular blood transfusions to stay alive.

His parents want him to undergo stem cell treatment that uses umbilical cord blood taken from a baby selected at embryo stage.

Lord Hoffmann said the technique of taking a single cell from an embryo and testing it for genetic disorders and tissue compatibility was "a way to save the Hashmi family from having to play dice with conception". The law lord said parliament had never stated that the testing of embryos to enable a mother to choose to carry a child with the characteristics of her choice was outlawed.

But Lord Brown said the ethical questions posed by the treatments were "profound". He asked: "Is this straying into the field ofdesigner babies or, as the celebrated geneticist Lord Winston put it, 'treating the offspring to be born as a commodity'?" He added: "In the unlikely event that the authority were to propose licensing genetic selection for purely social reasons, parliament would surely act at once to remove that possibility."

Mrs Hashmi said: "It has been a long and hard battle for all the family and we have finally heard the news we wanted to hear."