We would see this happening only in very rare circumstances and under strict controls

Ruth Deech, HFEA

However, it is possible to take tiny samples from the developing embryos and test them for the presence of a gene which would cause serious disease - such as cystic fibrosis, and then only use unaffected embryos, which would ensure a child free from that disease.

This is called preimplantation genetic diagnosis (PGD). Thalassaemia is one of the conditions for which PGD can be carried out.

In this case, the tiny samples would also be tested to find out which of them would be compatible with Zain - then only these would be used.

If a child is born, cells from the umbilical cord can be used as a transplant.

Case by case

The HFEA, which regulates the use of IVF embryos in the UK, has given the Hashmis the green light, but says any future applications will have to be assessed on a case-by-case basis.

And only those embryos already being tested for genetic flaws could be tissue typed as well, it says.

We dispute whether it is in the best interests of the child to be created as a tissue match for somebody else

Josephine Quintavalle, Prolife Alliance

Ruth Deech, who chairs the authority, said: "We have considered the ethical, medical and technical implications of this treatment very carefully indeed.

"Where preimplantation genetic diagnosis is already being undertaken we can see how the use of tissue typing to save the life of a sibling could be justified.

"We would see this happening only in very rare circumstances and under strict controls."

Limited circumstances

The effect of this ruling is to allow tissue typing of embryos - but only where PGD is also an option.

Other potential cases - for example, a couple who want to have a matched baby because their child has leukaemia and needs a bone marrow transplant - would not be covered by this ruling, because there is no PGD test for leukaemia.

The decision has infuriated pro-life campaigners, such as the Pro-Life Alliance.

Josephine Quintavalle, from the alliance, told BBC News Online: "The HFEA has to take the welfare of the child into account when they are making decisions like this.

"We dispute whether it is in the best interests of the child to be created as a tissue match for somebody else."

And fertility expert Professor Robert Winston, from Hammersmith Hospital in London, said that the ruling was wrong.

He said: "The trouble really is that this child as it grows up has been brought into the world because it is a commodity."

He said he feared the child would be obliged to give other stem cells to a sibling later in life - and said doctors should be focusing on finding cures rather than a solution which he felt had little chance of working.

Marrow concerns

However, the British Medical Association gave guarded approval to the ruling.

Dr Michael Wilks, chairman of its Ethics Committee, said: "We recognise that the HFEA ruling will be helpful to couples in a very limited and restricted number of cases."

But he added: "The BMA sees moral and practical differences between using umbilical cord blood for treatment which involves no discomfort or risk to the child and using the child him or herself as a donor which would involve physical risk to the child."