A ban on a couple creating a baby to help save their child has been overturned by the Court of Appeal.
Zain Hashmi is seriously ill
Four-year-old Zain Hashmi has the rare blood disorder thalassaemia and requires a bone marrow transplant.
He can be treated with blood transfusions but, over time, the amount of iron in the body can build to dangerous levels.
His parents, Raj and Shahana, want doctors to use pioneering IVF technology to screen embryos to find one that will provide a perfect match for him.
The family will now be able to go ahead with treatment "without delay" after the general ruling on Tuesday by Lord Phillips, the Master of the Rolls, Lord Justice Schieman and Lord Justice Mann.
This is a legitimate use of these new techniques
Shahana Hashmi said the family was "absolutely thrilled" with the court's decision.
She added: "We have said all along that at the centre of this case was our son, a little boy who suffers greatly.
"We are also delighted because this case opens the door to other families who are suffering.
"Whether or not we succeed, this decision has given Zain and us new hope.
"We feel that we should never have had to go through this, but it has made us all stronger and shows that justice can be done."
The Appeal Court decision overturns a High Court ruling in December last year which said the Human Fertilisation and Embryology Authority (HFEA) did not have the power to license the technique under existing legislation.
It followed a challenge by
Josephine Quintavalle, of the public interest group Comment on Reproductive Ethics (Core).
It is a defeat for society at large and certainly an overwhelming defeat for Parliamentary democracy
Josephine Quintavalle, Core
The ruling will allow other families in a similar situation to the Hashmis apply to be able to use the tissue-typing technique.
But the HFEA said strict regulations were in place and the decision would not open the floodgates for babies "designed" for social reasons, such as eye colour.
Suzi Leather, chair of the HFEA, said: "We are pleased that the Court of Appeal has upheld our decision.
"This means that the Hashmi family can continue with their treatment.
"Clearly clinicians cannot always prevent diseases but if they are able to and also save the life of a sibling, then this is a legitimate use of these new techniques."
The British Medical Association also backed the Appeal Court's decision.
'Case by case'
Dr Simon Fishel, of Nottingham's Centres for Assisted Reproduction, who is treating the family, said: "I am absolutely delighted for the Hashmis in particular and all our other patients who wish to remain in the UK for their treatment.
The Hashmis say they are simply trying to help their son
"I am relieved that this judgement, once and for all, supports the HFEA as the proper regulatory body for licensing these technologies."
He added: "From the public's point of view they should have no fear because cases such as the Hashmis and the procedures involved will remain highly regulated by the HFEA and strict conditions will apply to all couples seeking this treatment on a case by case basis."
Josephine Quintavalle, spokeswoman for Core, said: "We extend yet again our well wishes to the Hashmi family, against whom we took no legal action whatsoever."
But she added: "There are serious issues at stake here and from that perspective it is a defeat for society at large and certainly an overwhelming defeat for Parliamentary democracy."
The Hashmis have sought a bone marrow donor for their son, but no genetic match has so far been found.