A week-old conjoined twin died and her sister has a 50-50 chance of survival after they were separated by emergency surgery, a hospital said today.
Hope Williams died last night after medical staff at Great Ormond Street Hospital, in central London, carried out "one of the most complex and challenging" operations they have ever faced.
Professor Agostino Pierro, head of the surgical team, said Hope's lungs proved too small to support her breathing.
He said twin Faith was in a "stable" condition and "gradually improving".
The twins, who were joined from the breastbone to the top of the navel and had separate hearts, were delivered by Caesarean section at London's University College last Wednesday and taken to Great Ormond Street.
Their mother, 18-year-old Laura Williams, from Shrewsbury, Shropshire, was Britain's youngest mother to give birth to conjoined twins.
Medical staff warned the babies might not survive after a 12-week scan revealed the problem, but Mrs Williams and her husband Aled, 28, refused to consider a termination.
Prof Pierro said his team had to start operating last night because of a "sudden deterioration" in the children's health.
"This was an emergency operation because there was a blockage in their joined intestine which could only be resolved through separation.
"The technical surgery worked well - although it was extremely challenging - and went according to plan."
He added: "However, very sadly, after separation baby Hope's lungs proved too small to support her breathing and she died last night in the presence of her parents.
"They are clearly devastated by the loss of their daughter and we offer them our deepest condolences on their loss.
"Baby Faith is stable after separation. She requires support for her breathing but she is gradually improving. However, it is early days and complications can occur."
More than 20 hospital staff were involved in the procedure and the twins were moved into different theatres after they were separated.
Prof Pierro told reporters outside Great Ormond Street Hospital that Faith had a 50:50 chance of survival following her operation, which continued after Hope died.
The procedure lasted for about 11 hours in total and finished at 7pm last night.
He said: "If everything goes well, there will be more surgery required to close her tummy but I can't tell you now when the surgery will happen.
"It's fair to say the operation done on Hope and Faith was one of the most complex and challenging we have ever faced."
Before the operation, the twins' parents spoke exclusively to the Mail on Sunday about the birth.
Mrs Williams told the newspaper: "They wheeled me in to see them.
"They had tucked Hope's arm underneath and it was Faith's arm that I could see. I took her hand and she was grasping my hand.
"They were both blowing bubbles. They were so beautiful. After everything everyone said, I'm so glad to have proved them wrong."
Mr Williams said he was "over the moon" after the birth.
The couple have another daughter, 18-month-old Carly.Reuse content