Elton John and Princess Diana were good friends, leaning on each other during difficult times and working closely together on causes they were both passionate about.
Meeting at Prince Andrew's 21st birthday party before Diana and Charles's Royal Wedding, the pair enjoyed years of friendship before a sudden and "completely unexpectedly" fall out.
Speaking openly in his new book, Elton explains that he believes Buckingham Palace may have been behind their disagreement.
He says they had a row when Diana pulled out of charity project they were both due to be part of - a book by fashion designer Gianni Versace.
Diana was meant to be writing the forward for Rock And Royalty, a collection of photographs published to raise cash for the AIDS Foundation.
Elton writes: "Then she got cold feet. I think Buckingham Palace didn't like the idea of a member of the Royal Family having anything to do with a book that featured shots of naked guys with towels draped around them."
He says Diana withdrew at the last minute, claiming she didn't know about the book's contents - which he says was untrue.
"I wrote back to her, calling her out, telling her how much money she had cost the AIDS Foundation, reminding her that she had seen the book."
She wrote back, but the reply was very formal and "several" - starting 'Dear Mr John.'
But he adds that despite his anger he was very worried about his friend at the time.
He writes: "I was angry with her, but I was also worried. She seemed to be losing touch with all sorts of really close friends, who would be honest with her, and surrounding herself instead with people who told her what she wanted to hear.
"I knew from personal experience that wasn't a healthy situation."
Diana and Elton didn't speak again until the day Versace died.
She called him at his holiday home in Nice, apologising for their fall out - saying "I'm so sorry. It was a silly falling-out. Let's be friends."
They went to the funeral together and remained friends until Diana's death.
Elton sang Candle In The Wind at her funeral, which he originally recorded in 1973 in honour of Marilyn Monroe and reworked for Diana.
It became one of the fastest selling singles in the country, with 658,000 people buying it on the day it was released.
- Me: Elton John Official Autobiography is out on October 15.
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