Parents who are trying to have a 'genetically selected' baby to help save the life of one of their sick children have warned that they will appeal to the European Court of Human Rights if the House of Lords this week bans the creation of 'saviour siblings'.

Last night Shahana and Raj Hashmi told The Observer that they had begun talks with human rights lawyers in case the Lords decides to ban the creation of babies selected to be sources of cells and transplants for sick older brothers and sisters. If necessary, they will travel abroad to have the treatment.

'We will certainly take action if this hearing goes against us,' said Shahana Hashmi, who is trying to have a 'saviour sibling' to provide bone marrow for her sick son, Zain. 'I am certainly not going to stop trying to have a brother or sister who could save Zain.'

The Lords will begin its sittings on the issue tomorrow and will announce its decision in the near future. It will be one of the most eagerly awaited judgments on the delicate subject of genetic medicine and human life.

At issue is the legality of the actions of the Human Fertility and Embryology Authority, which recently announced that it was going to allow parents to proceed with the creation of saviour siblings. Under this system, they could pick one of a range of embryos created by IVF techniques, so that its tissue would match that of an elder sibling. If that sibling is affected by a debilitating genetic disease, the younger child could then be used as a source of life-saving transplants or transfusions.

In the case of the Hashmis, their son, Zain, has beta-thalassaemia and his parents want to have a brother or sister whose tissue is compatible with his. The new sibling could then provide bone marrow for a transplant that would give him a normal healthy lifespan. At present, Zain needs blood transfusions every three weeks to keep him alive.

But the principle behind this technique is opposed by pro-life groups, who object to the creation and subsequent destruction of unwanted embryos that are involved in creating saviour siblings. The organisation Comment on Reproductive Ethics mounted a legal challenge to the practice, claiming that the HFEA had acted beyond its authority in giving approval to the creation of saviour babies.

'The actions of the authority are the most drastic of deviations it has yet made from its legal remit,' said Josephine Quintavalle, who runs the group.

'They have backed a technique that allows embryos to be created and destroyed purely because their tissue does not match what a parent wants. This is a slippery slope. Once you start doing that, you could start to create and destroy embryos for all sorts of relatively trivial reasons. I am not trying to stop any individual from attempting to help a sick child.

'This is not an issue about individuals,' added Quintavalle. 'What concerns me is the fact the HFEA has gone far beyond its legal authority. If we are going to allow saviour siblings to be created here, then it is for Parliament to decide, not some obscure group like the HFEA.'

The group's objection was backed in court, but subsequently thrown out by the Court of Appeal.

Now the case is to be heard by the House of Lords this week. If it backs the appeal court, the implications for 'designer baby' technology will be profound.

Families like the Hashmis will find it impossible to continue their quest to find a sibling to save Zain in this country. 'Obviously, it would be a blow,' said Mrs Hashmi. 'However, we have already had talks with human rights lawyers about taking this to Europe and, if that does not work, then we will simply go abroad to get the treatment.'

In the past, other UK families have taken this approach and now have children whose lives have been saved by younger brothers and sisters who were created by IVF in US hospitals.