A baby girl remains seriously ill in hospital after her conjoined twin died following emergency surgery to separate them.
Hope Williams died on Tuesday night after "one of the most complex and challenging operations" ever faced by medical staff at Great Ormond Street Hospital for Children, in London.
Surgeons have given her sister Faith a 50-50 chance of survival, and she is still in a stable condition in intensive care.
Professor Agostino Pierro, head of the surgical team, said Hope's lungs proved too small to support her breathing after the separation.
The twins were delivered by Caesarean section at London's University College Hospital last Wednesday with a combined weight of 4.8kg (10.5lb), and transferred to Great Ormond Street.
They were joined from the breastbone to the top of the navel and had a shared liver but separate hearts.
The girls' mother, 18-year-old Laura Williams, from Shrewsbury, Shropshire, is Britain's youngest mother to give birth to conjoined twins.
The condition, which is extremely rare, occurs when the single egg from which identical twins develop fails to divide properly after conception.
Medical staff warned Mrs Williams and her husband Aled, 28, that their babies might not survive after a 12-week scan revealed the problem but they refused to consider a termination.
Doctors had hoped to wait until the twins were older and stronger to operate, but Prof Pierro said his team had to begin surgery on Tuesday because of a "sudden deterioration" in the children's health.
