One of the conjoined twins born last week has died and her sister was critically ill last night after emergency surgery to separate them.

Hope Williams died on Tuesday night after doctors decided to carry out the operation sooner than planned because the sisters’ shared intestine had become blocked.

The surviving girl, Faith, was described as being in a critical condition with a 50-50 chance of survival after the 11-hour operation.

The twins’ parents, Laura and Aled Williams, were at their bedsides when Hope died at Great Ormond Street Hospital for Children in London.

Mrs Williams, 18, from Shrewsbury, had become the world’s youngest mother of conjoined twins. She and her husband had refused to have the pregnancy terminated after doctors said that the twins would probably not live beyond two weeks.

Professor Agostino Pierro, head of the surgical team that separated the sisters, said Hope’s lungs had proved too small to support her breathing.

He said 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 University College Hospital last Wednesday and taken to Great Ormond Street.

Professor Pierro said that his team had to start operating 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,” he added. “The technical surgery worked well — although it was extremely challenging — and went according to plan. 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. Professor Pierro said that Faith had a 50-50 chance of survival after the operation, which continued after Hope died. “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,” he said. “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 Mrs Williams had said: “We knew that conjoined twins very rarely make it through the first 24 hours, and we could see that one girl was a bit smaller than the other, so we called the little one Hope and the bigger one Faith.”

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