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.

Hope Williams died on Tuesday 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. The couple have another daughter, 18-month-old Carly.

Prof Pierro said his team had to start operating because of a "sudden deterioration" in the children's health.

More than 20 hospital staff were involved in the procedure and the twins were moved into different theatres after they were separated.

The procedure lasted for about 11 hours in total.

-Nova