Lucy O'Donnell was a childhood friend of John Lennon's son Julian, and the song title was inspired by a picture that he had drawn of her at school.
"That's Lucy in the Sky with Diamonds," Julian explained to his father when he took the picture home.
Many fans believed that the classic 1967 hit, recorded for the "Sgt. Pepper's Lonely Hearts Club Band" album, was a thinly disguised paean of praise for the hallucinogenic drug LSD.
"I remember Julian and I both doing pictures on a double-sided easel, throwing paint at each other, much to the horror of the classroom attendant," O'Donnell told BBC radio in 2007.
"Julian had painted a picture and on that particular day his father turned up with the chauffeur to pick him up from school."
The St Thomas' Lupus Trust charity said O'Donnell had died aged 46 from the autoimmune disease lupus.
It said Julian and his mother Cynthia, Lennon's first wife, were "shocked and saddened by the loss of Lucy."
"It's so sad that she had finally lost the battle she fought so bravely for so long," said Angie Davidson, the Trust's campaign director.
(Reporting by Michael Holden; Editing by Steve Addison)
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