The final legal barrier to the creation of "saviour siblings" to treat children seriously ill with genetic disorders was swept away by the Law Lords yesterday.

In a landmark case, the House of Lords ruled that the use of modern reproductive techniques to create babies to be used to treat siblings could be authorised by the Human Fertilisation and Embryology Authority.

The decision delighted scientists and marked a victory for the couple at the centre of the case, Raj and Shahana Hashmi, who are hoping to have a baby with the same tissue type as their son, Zain, to treat his rare blood disorder.

Mrs Hashmi said : "It's nice to know that society has now embraced the technology to cure the sick and take away the pain. It has been a long and hard battle for all the family and we have finally heard the news we wanted to hear.

"We feel this ruling marks a new era and we are happy to move forwards now. We hope and pray that we get what we need for Zain. He has been our inspiration throughout all of this. We haven't got the words to say how much we have appreciated everyone's support through this time, 'thank you' doesn't seem enough."

The test case centred on an Appeal Court judgement in April 2003 which overturned a ban on the use of the controversial treatment to save the couple's terminally ill son.

Zain, 6, suffers from thalassaemia, an inherited blood disorder and needs blood transfusions every three weeks. He might be cured with a stem cell transplant but no matching donor has been found.

The Hashmi's had fertility treatment in which their embryos were tested and tissue typed to match Zain before being replaced in Mrs Hashmi's womb. But the attempt failed when Mrs Hashmi, now 38, miscarried.

The couple's efforts to produce a saviour sibling were temporarily blocked by Josephine Quintavalle and her campaigning group, Comment on Reproductive Ethics, who took the case to the Law Lords, after the HFEA approved the treatment.

Ms Quintavalle claimed that the HFEA had acted beyond its powers and the creation of "designer babies" was against the law.

Yesterday five Law Lords who heard the case last month ruled unanimously that the practice of tissue typing to create babies to help siblings could be authorised by the HFEA.

Lord Hoffman, giving the lead judgment, said the case centred on the use of diagnostic and tissue typing techniques to determine whether an embryo would be suitable for the purpose of being placed in Mrs Hashmi.

Ms Quintavalle had claimed that this gave far too wide a meaning to the notion of being "suitable" and would enable testing for sex, hair colour and intelligence.

Lord Hoffman said Parliament had never stated that the testing of embryos to enable the mother to choose to carry a child with the characteristics of her choice was outlawed.

"One infers that the White Paper intended the fundamental ethical issues which such activities might raise to be determined by the statutory authority, subject to the regulation-making power by which Parliament could impose it own decision," he said.

Lord Brown of Eaton-Under-Heywood said the ethical questions posed by the treatments were "profound".

He had at first supported the argument that tissue typing was impermissable but once the concession was made that screening for defects should be licensed, there was "no logical basis" for ending the authority's power at that point.

"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."

The Hashmi's doctor, Dr Simon Fishel, of the CARE Fertility Group, paid tribute to the family.

"I trust that a line has now been drawn in our society whereby individual families, with guidance from their medical practitioners, can make their own choices without extraordinarily long and compromising legal battles. Now it is time to move on."