The fertility watchdog gave the go-ahead yesterday for parents to create "designer babies" who can act as genetically-matched donors for their sick siblings.

In a controversial policy change, the Human Fertilisation and Embryology Authority (HFEA) decided to relax the rules governing the screening of embryos before they are implanted in the womb. Experts predict that around 20 couples a year may now use the procedure to create "donor siblings".

But anti-abortion groups warned the policy change could lead to babies being created as "spare parts" rather than as human beings in their own right.

Over the past three years, several applications to the HFEA have been made by couples who want to use pre-genetic diagnosis (PGD) screening to help them select embryos which will be a genetic match and donor for existing children suffering from life-threatening diseases and whose only hope is a stem cell transplant.

Only one couple has been allowed to use the procedure so far and fertility experts have complained the existing guidelines are outdated and unethical.

Announcing the policy change Suzi Leather, the HFEA chairwoman, said: "Faced with potential requests from parents who want to save a sick child, the emotional focus is understandably on the child who is ill. Our job is to consider the welfare of the tissue-matched child which will be born.

"Our review of the evidence does not indicate that the procedure disadvantages resulting babies compared to other IVF babies. It also shows that the risks associated with sibling-to-sibling stem cell donation are low and that this treatment can benefit the whole family."

One of the first people to benefit from the policy change will be two-year-old Joshua Fletcher, who suffers from a life-threatening blood disorder.

Joshua was born with Diamond-Blackfan anaemia, a blood disorder affecting just 100 children in Britain. He needs blood transfusions every three weeks and will soon have to start enduring painful injections into his stomach three times a week to prevent complications from his condition.

The only cure is a transfusion of stem cells - the building blocks of the body - to stimulate Joshua's production of healthy red blood cells. Neither Joshua's parents Joe and Julie, nor his five-year-old brother Adam are close enough genetic matches, so the Fletchers want to create up to 12 embryos through in vitro fertilisation.

Scientists will then screen the embryos to find ones that match Joshua's tissue type and genetic matches will be implanted into Mrs Fletcher, in the hope she will become pregnant. At birth, stem cells will be removed from the baby's umbilical cord in a painless procedure, and then transplanted into Joshua.

Mr and Mrs Fletcher, from Co Down, Northern Ireland, insist they have always wanted another baby and are not creating a child as a commodity. Mr Fletcher, 36, said: "We only want to give our son the best chance of a cure for a condition which could take his life."

Fertility experts welcomed yesterday's ruling.

Simon Fischel, who heads the Care fertility unit at Park Hospital in Nottingham, said: "I am extremely pleased as this is the right decision. I think that the previous regulation was ethically untenable in our society."

Dr Fischel has been treating Raj and Shahana Hashmi, the couple from Leeds who last year were the first in Britain to be given permission by the HFEA to use PGD in an attempt to create a donor sibling for their seriously-ill son Zain.

Mrs Hashmi has gone through several cycles of IVF and has become pregnant twice, but miscarried on both occasions. On the advice of doctors, she has stopped IVF treatment, but still has a few screened embryos left which could be used if the couple wanted.

Two years ago, Jayson and Michelle Whitaker were refused permission by the HFEA to use PGD to create a genetically-matched sibling for their son Charlie, who also suffers from Diamond-Blackfan anaemia. They went to the America for the treatment and last year Mrs Whitaker gave birth to a son, Jamie.

Cells from Jamie have been transplanted into Charlie and doctors are waiting to see whether the procedure has been successful.

But anti-abortion groups condemned the ruling and said donor-siblings could be emotionally damaged by the knowledge that they were created to cure their brother or sister.

A spokesman for Life, which campaigns against abortion, cloning and other scientific procedures, said: "To attempt to create another child as a transplant source is not morally acceptable.

"How would this child feel, for example, when he or she discovers that they were brought into the world primarily as a "spare part" for their elder brother? Human beings - particularly children - must never be used as a means to an end."