One of the conjoined twins born six days ago in London has died, after an operation to separate the two.

Hope Williams died after 12 hours of surgery at Great Ormond Street Hospital which also left her sister Faith critically ill, it has been revealed.

Both girls made it through the separation operation but Hope's lungs were too small to support her breathing.

Her parents Laura and Aled Williams were at her bedside when she died Tuesday. Doctors said the couple are devastated but are now praying Faith will pull through.

Laura, 18, had made medical history by going through with the birth to become Britain's youngest mother of Siamese twins.

Faith is breathing with the help of a ventilator and her condition is stable, although Professor Agostino Pierro, who led the surgical team, warned it was still early days.

Outside the hospital this morning, he said her chances of survival were around 50-50 but that she would face more surgery if she continued to improve.

'It's fair to say the operation done on Hope and Faith was one of the most complex and challenging we have ever faced,' the surgeon said.

He revealed both twins could have died had they not performed the emergency surgery because of a blockage in their shared intestine.

The separation surgery, which involved a team of 20 experts, only finished at around 7am this morning and 'went according to plan'.