The creation of "designer babies" to treat sick siblings is lawful, the Law Lords have ruled, upholding an earlier court decision.
The right donor could cure Zain
The case centred on six-year-old Zain Hashmi, whose parents wanted a baby with a specific tissue type to help treat his debilitating blood disorder.
His parents had begun treatment to create a baby, but have so far failed.
Campaigners had asked the Lords to overturn the appeal court's 2003 ruling that allowed the couple to proceed.
The group Comment on Reproductive Ethics (Core) asked the House of Lords to examine the Human Fertilisation and Embryology Act 1990 and to decide whether tissue-typing of the sort used by the Hashmis was legal.
On Thursday, five Law Lords ruled unanimously that the practice of such tissue typing could be authorised by the Human Fertilisation and Embryology Authority (HFEA).
The ruling, saying that HFEA was acting lawfully and appropriately in considering and granting a licence for pre-implantation tissue-typing, was welcomed by the authority.
"We are pleased with the clarity that this ruling brings for patients," the HFEA said.
Mrs Hashmi said the family appreciated the support they had received throughout the legal process.
"It's nice to know that society has now embraced the technology to cure the sick and take away the pain.
"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."
To stay alive Zain, who suffers from beta thalassaemia major, currently has to have blood transfusions every month, and drugs fed by a drip for 12 hours a day.
Technology now allows doctors to select embryos with perfect tissue for a transplant operation.
The High Court had imposed a ban on the treatment in December 2002 but this was overturned in the Court of Appeal.
That decision allowed parents Raj and Shahana to go ahead with treatment to produce a sibling with the same tissue type as their son.
In theory, this would have allowed them to take stem cells from the new baby's umbilical cord and transplant them into Zain.
Tragically, however, Mrs Hashmi has had a series of miscarriages.
The ruling "saddened" anti-abortion campaigners Life.
"Today's decision from the House of Lords takes us further down the slippery slope in creating human beings to provide spare parts for another.
"The best of ends, namely to cure a sick child, does not justify the means."