Conjoined twin Faith Williams has lost her battle for life, four weeks after she was born.
Faith survived surgery to separate her from twin sister Hope but died yesterday "from the complexities of her condition".
Professor Agostino Pierro, head of Great Ormond Street Hospital's surgical team, said: "She required the full range of skills of our intensive care staff, and underwent a number of further procedures.
"However, she succumbed to the complexities of her condition."
The twins were born on November 26. Hope died following surgery to separate them at the beginning of the month. They were joined from the breastbone to the top of the navel and had a shared liver but separate hearts.
Their parents Aled Williams, 28 and mother Laura, 18, from Shrewsbury, Shropshire, maintained a constant vigil at their bedside. Mrs Williams is Britain's youngest mother to give birth to conjoined twins.
Prof Pierro said: "While this is a sad outcome, it is not an unexpected one."
Faith underwent surgery earlier this month to assist with her circulation. "The aim was to ensure more blood flows to her body and less through her lungs," a spokeswoman for the hospital said.
Following the heart surgery the baby had another operation to close her chest. Hope failed to survive the initial operation to separate her from Faith because her lungs were too small to support her breathing.
A hospital spokesman said: "Mr and Mrs Williams have asked us to say that they were very happy with the care they and their children received at Great Ormond Street."