A five-year-old from Gloucestershire has lost her battle with a brain tumour days after fulfilling her ambition to sing for Simon Cowell.
Bethany Fenton, who performed Twinkle Twinkle Little Star for the Britain's Got Talent judge on Saturday, died on Tuesday with her family at her side.
Bethany, from Fairford, fought the illness for over a year and was said to be "over the moon" at meeting the star.
Cowell's spokesman Max Clifford said: "He is really upset."
Mr Clifford said: "He said what a beautiful little girl she was.
"He is just so sad, and sad for the whole family. She was a very special little girl and Simon was so touched when she sang for him.
"He was happy that he was able to do a special something on Saturday night to give her a few precious moments but it is so sad and his heart goes out to Bethany's mum and dad and he is thinking of them."
Bethany also met the Britain's Got Talent finalists in the special trip at the weekend.
Her mother Jemma and father Sean, a PE teacher, are "devastated" by her death, family spokesman Robin Thompson said.
Mr Thompson added: "They came back from Britain's Got Talent on Saturday. On Sunday she was fine, she was absolutely over the moon with what happened.
"On Monday morning she started being very poorly and the illness went on throughout the night and reached a bad stage this morning."
She was taken to the John Radcliffe Hospital, in Oxford, for treatment but died later.
In 2009 Bethany's family was told by surgeons in America that there was "no hope" for her. She had also received radiotherapy and chemotherapy.
A fund-raising campaign, which had been set up to try to raise the £50,000 needed to send her for treatment at a specialist clinic in the USA, was put on hold last September.
Speaking then, Mr Thompson said: "In the first five days we've had £10,000 given or promised in cash, 30 [fundraising] events lined up, and had heard someone had telephoned anonymously and offered all £50,000 [required for Bethany's treatment]."