Medical treatment to produce so-called "sibling saviours" is against the law, the House of Lords was told today.

Five law lords are hearing a test case which if successful would halt the practice of tissue typing to create babies to help treat siblings with genetic disorders.

The case chosen involves six-year-old Zain Hashmi who was born with beta thalassaemia - a serious and potentially fatal genetic disorder.

His parents, Raj and Shahana, want him to undergo stem cell treatment using umbilical cord blood taken from a baby they would produce using IVF treatment that had been especially selected at embryo stage so that the tissue matched Zain's.

They were able to begin treatment after an appeal court ruling in April 2003 overturned a ban on the use of the controversial treatment. Their first attempt ended in a miscarriage.

Now a campaigning group, Comment on Reproductive Ethics, is seeking to outlaw the practice.

Lord Brennan QC, representing the group, told the law lords they must decide whether the law allows the Human Fertilisation and Embryology Authority to grant a licence to permit tissue typing to test for compatibility.

"It is our case that the authority does not have the power to licence tissue typing."

He said the treatment was a method of selecting from healthy embryos those that if implanted into the mother's womb produce a child whose tissues match that of a person who needs treatment.

"This raises serious ethical issues," he said. "We are dealing with difficult and sensitive questions."

The court of appeal was wrong in concluding that tissue typing was a treatment service under the Embryology Act because the service was not done for the purpose of helping a woman to have a child.

Lord Brennan said the embryo is chosen not for the benefit of that potential child but for the benefit of another person.

The hearing continues.