Molly Nash cradles her genetically selected brother, Adam

Molly received cells from her brother's umbilical cord to help her fight the inherited bone marrow disorder, which can cause bleeding, immune system problems and leukaemia.

The Nottingham centre's director, Simon Fishel, said the two families had broached the subject with him and they were waiting for medical referrals.

"If we get the medical referral, that would trigger an application to our ethics committee, then to the Human Fertilisation and Embryology Authority," he said.

"If the HFEA decide that the Nash scenario is ethically acceptable, we could get a licence within two months of applying.

Long wait

"But at the moment, they are in consultation which can take a year or more. It really depends on the HFEA.

"If the families have got the means and if their child is very sick, then maybe they should consider America.

"The issue here is that you need to get it available in this country as soon as possible."

The specialist said that using stem cells from the umbilical cord in certain circumstances, such as when an older child is critically ill, could help hundreds of people in similar situations.

Ongoing review

"My personal belief is that the HFEA will licence it, but not a scenario using embryos as a commodity," he said.

A spokesman for the HFEA said the organisation was currently carrying out a review of the uses of pre-implantation genetic diagnosis.

It has so far only been used to screen for often life-threatening illnesses.

"We are not currently considering (screening embryos for exact cell types to help sick older siblings) as an issue but obviously it has just come up so we may have to in the future," the spokesman said.