The hopes of dozens of families who want to produce a "saviour sibling" for their gravely ill child rest on the outcome of a case which goes to Britain's highest court today.

The Comment on Reproductive Ethics (Core) is challenging the Human Fertilisation and Embryology Authority's right to license doctors to screen embryos before implantation to try to create a baby who could be a blood or bone marrow donor.

The HFEA, which regulates in-vitro fertilisation in Britain, originally lost the case in the high court, where a judge ruled in December 2002 that it had exceeded its powers.

The court of appeal overturned the ban in April 2003, opening the way for Shahani and Raj Hashmi, the first couple granted a licence to use the technique, to continue trying to conceive a match for their son Zain, who has a serious blood disorder.

If Core wins its appeal to the law lords, the Hashmis, from Leeds, and dozens of other families will have to abandon their attempts to have a saviour sibling in Britain.

Mrs Hashmi, who has three other children, wants another child so its umbilical cord blood can provide stem cells to cure Zain, six, of beta thalassaemia. She has had six attempts so far, resulting in five miscarriages and one occasion when none of the embryos was a close enough match.

Zain has to have an infusion of drugs five nights a week for 12 hours, and blood transfusions every three to four weeks.

Mrs Hashmi said yesterday she knew of more than 100 families who were hoping to have the treatment. She said the couple would take the case to the European court of human rights if Core won.

The appeal court judges ruled that parliament had left the decision to the HFEA, but Core argues that screening embryos genetically is "ethically objectionable" and should be decided by parliament.

The hearing is expected to last two days. The law lords normally take at least six weeks to deliver judgments.