In his upcoming memoir, Not Dead Yet, the singer describes the moment he spotted the pair in a car together in London, 1991, six years before Diana's tragic death.
"A BMW pulls up and the window slides down," the 65-year-old writes. "It's Diana and, sitting in the driver's seat, an officer-class chap I recognized as James Hewitt."
After some quick conversation, things took a turn for the uncomfortable, as Diana told Collins about a recent medical procedure she had undergone.
"'Light as you like,' she said 'I've just had a colonoscopy. It was great. You should try it.' Danny and I look at each other. 'Did that really just happen?'"
Rumors have circulated that Hewitt is the biological father to Diana's youngest son Harry, but the 58-year-old has staunchly denied the reports.
Diana's affair with Hewitt lasted for five years, as husband Prince Charles began his own extramarital relationship with now-wife Camilla Parker Bowles, Duchess of Cornwall.
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