A couple from Leicester have been given permission by the fertility watchdog to have a "saviour sibling" in a bid to help their sick 20-month-old daughter.
Charlotte has needed hospital treatment
Charlotte Mariethoz has the rare blood condition diamond blackfan anaemia and doctors say she needs a stem cell transplant from a genetic match.
Her parents will now use IVF to try to have a "designer baby" to save her.
In 2004, a British boy with the same disorder was saved by cells from a baby brother born after embryo selection.
Michelle and Jayson Whitaker from Chesterfield had Jamie, a saviour sibling for their son Charlie after flying to the US to have the treatment there.
Charlotte has already undergone a number of blood transfusions, and is at risk of further complications including a risk of developing a form of leukaemia.
Without a transplant she would need further blood transfusions and steroids, her parents Catherine and Charlie have been told by doctors.
Mrs Mariethoz said: "This is the first step in the right direction. Obviously there is a long way to go yet.
"We hope this decision will now help other families in similar situations."
The Human Fertility and Embryology Authority (HFEA) has issued a limited number of licences for similar treatments before.
But critics say "designing" a child as a tissue donor for a sick sibling undesirable and unnecessary.
Tissue testing involves taking one or two cells from an embryo when it is around three days old.
If there is a match for the existing child, that embryo will be implanted in the mother's womb so it can develop.
Once a baby is born, stem cells are removed from the umbilical cord and stored for four to six months when the transplant to the sick child can take place.
Charlotte's condition is not genetic, so there is very little chance any sibling will also be affected.
But Dr Simon Fishel, who is treating the Mariethoz family, says using tissue typing will allow a child who is a genetic match for Charlotte to be conceived.
"The family have been told that a non-related stem cell transplant is not acceptable, so maybe they should have more babies.
"And it would be much more devastating to try naturally and find out they haven't got a match and can't help Charlotte."
He said, if the treatment is successful, it would be the first time a saviour sibling would be known to have been conceived and born in the UK.
Last year, the parents of six-year-old Zain Hashmi who requires treatment for the blood disorder betathalassaemia major, won permission to seek to create a saviour sibling from the House of Lords, after originally being refused a licence by the HFEA.