The UK's fertility watchdog is to relax the rules on the creation of so-called "designer babies" to help sick siblings.
Some argue current policies are inconsistent and confusing
It says embryos can be selected which are free of disease and which can provide blood cell transplants to treat sick brothers and sisters.
Until now there have been strict restrictions on the use of the technology.
But the Human Fertilisation and Embryology Authority's decision has been called scandalous by critics.
Suzi Leather, head of the HFEA, said: "We have decided to relax the rules on embryo selection to enable all couples who want to be able to select an embryo who might be a tissue match for an existing seriously ill sibling to be able to do that."
She stressed each case would be looked at on its own merits, and said it would be a "treatment of last resort."
But Josephine Quintavalle, of the group Comment on Reproductive Ethics, said: "We are absolutely appalled. It is grossly unethical, and grossly undemocratic.
"This decision has been taken behind closed doors without any consultation with the public. The HFEA, who are unelected and unrepresentative, think they have the expertise to make a decision on behalf of the nation."
And Professor Jack Scarisbrick, national chairman of the charity Life, said: "We have gone yet further down the slippery slope in creating human beings to provide 'spare parts' for another."
'Give him a chance'
The technique involves taking a cell sample from the embryo at around the eight cell stage.
It allows a doctor to test tissue type to find a genetic match for an existing child.
The HFEA had previously restricted the technique's use because of concerns about a potential risk of damaging the embryo.
Families have only been able to use it if they are affected by genetic conditions and the embryo itself is being screened to ensure it will not be affected.
But the HFEA says it has now been able to review more evidence on the medical, psychological and emotional implications for children and their families as well as the safety of the technique and is happy to give the go-ahead for the rules to be relaxed.
Dr Simon Fishel, Managing Director of Centres for Assisted Reproduction said: "I have no doubt that this is the right decision.
"In the real world these families are often faced with trying to conceive a tissue-matched child through natural conception and this can result in numerous heart breaking terminations of pregnancy, the birth of children not tissue matched or further children with a life threatening disease.
"Parents have the right to choose technology to help them overcome their extraordinarily painful circumstances."
A spokeswoman for the British Medical Association welcomed the HFEA's decision: "If the technology to help a dying or seriously ill child exists, without involving major risks for others, then it can only be right that it is used for this purpose."
The HFEA's decision offers hope to the family of two-year-old Joshua Fletcher, from Moira, County Down.
Joshua has a potentially fatal blood disorder called Diamond Blackfan anaemia (DBA), which can be treated by using stem cells to stimulate his body to produce healthy red blood cells.
Neither his parents, Joe and Julie, nor his five-year-old brother, Adam, are close enough matches to give him the stem cells he needs.
Dr Mohamed Taranissi, the director of London's Assisted Reproduction Gynaecology Centre, is treating the family and backs a change in the rules.
He said: "What we are doing is simply helping treat sick children."