A couple banned from creating a baby to help their seriously-ill son are to ask the Court of Appeal to overturn the decision.
The Hashmis say they are simply trying to help their son
Four-year-old Zain Hashmi has the rare blood disorder thalassaemia and urgently needs a bone marrow transplant.
He can be kept alive through 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 court will be asked on Tuesday whether the screening can go ahead.
The High Court ruled in December last year that the Human Fertilisation and Embryology Authority (HFEA) did not have the power to license the technique under existing legislation.
The ruling followed a challenge by
Josephine Quintavalle, of the public interest group Comment on Reproductive Ethics (Core).
The HFEA is now returning to the court to try to get that decision overturned.
'Right to life'
Shahana Hashmi said: "We believe that we are doing is the right thing for Zain. He has a right to life.
"Any suffering he has can be alleviated through medical science.
Designing another child as a therapeutic commodity, as a tissue bank, is definitely not an ethical cure
Josephine Quintavalle, CORE
"We're going to do everything in our power to ensure that Zain has that right to life."
Zain is being kept alive through blood transfusions and a drug treatment to keep his iron levels down which has to be given for 12 hour periods, five nights a week.
The Hashmis have sought a bone marrow donor for their son, but no genetic match has so far been found.
The couple, who already have five children aged three to 18, want to have another baby to help them to save their son.
Shahana Hashmi said: "This is just another obstacle which we will overcome."
She added: "We draw our strength from Zain. He's the one who gives us the strength to carry on and do what we have to do."
Mrs Hashmi, 38, said Zain understands what is happening, and talks about his parents going to court to stop "the baddies" who are stopping him getting better.
She added: "He asks me 'when's my baby coming so I don't have to have my medicines any more'."
The Hashmis, from Leeds, are being treated by IVF expert Dr Simon Fishel, at The Park Hospital in Nottingham.
He told BBC News Online the moral and ethical considerations of the treatment had already been debated at his centre, and by the HFEA itself.
Zain Hashmi needs a bone marrow transplant
He said: "It was cruel to apply for a judicial review when society has already been involved in the process."
Suzi Leather, head of the HFEA, said it was returning to court: "not only for this family, but for other families who are suffering from similar genetic conditions."
The doctor treating the Hashmis said he knew of at least half a dozen families in the same position at his clinic alone.
Professor Vivienne Nathanson, head of science and ethics at the British Medical Association said it backed the HFEA stance.
But she added: "We would go further and say that not only should tissue typing be given to help the Hashmis, but also to help other families, not only with inherited diseases but where cord blood could help save a child."
But Josephine Quintavalle, spokeswoman for Core, said: "We have absolute compassion for Zain Hashmi and any baby born or unborn who suffers from the disease which afflicts him.
"We hope that ethical cures in stem cell therapies (or otherwise) will come quickly to the relief of all such sufferers. Work with unrelated cord-blood, for example, looks extremely promising.
"But designing another child as a therapeutic commodity, as a tissue bank, is definitely not an ethical cure. Please think for a moment about the rights of that tiny vulnerable baby."
The court hearing is expected to last for two days, with a decision expected by Easter.