A father who had to tell a surgeon to let his conjoined twin daughter die described it as "the most difficult choice of my life".
Aled Williams, 28, from Shrewsbury, Shropshire, said he thought his daughter Hope had suffered enough.
"They were still trying to resuscitate Hope and I thought it's not fair on her. Her body just couldn't take it," he told The Mail on Sunday.
Hope's sister Faith, who survived surgery to separate the twins, is still fighting for her life at Great Ormond Street Hospital in London, but doctors said on Thursday she remained "a very sick little girl".
Mr Williams said: "She must really want to be in this world because she's really fighting."
He described how the hospital had been wrapping her in clingfilm to keep her warm because she was so cold.
He and his 18-year-old wife, Laura Williams, from Shrewsbury, said that even though they had just lost one of their daughters, they had to stay strong for Faith.
They plan to give Faith the middle name Hope, in memory of her dead sister.
Mrs Williams became Britain's youngest mother to give birth to conjoined twins last Wednesday.
Hope failed to survive after the operation on Tuesday because her lungs were too small to support breathing, doctors said.
Her mother found it too difficult to say goodbye to her dead daughter, but her father held her in his arms after telling the hospital to turn off the medical equipment keeping her alive.