Back in May 1999, he revealed during a heartbreaking TV interview the moment he was given the news that Diana had been tragically killed in a car crash in Paris on 31 August 1997.
Speaking to US chat show host, Larry King, on CNN, Charles spoke of the moment he learned of the news.
"I was in Cape Town, South Africa with my four children, just the five of us in the house, and the telephone went very early in the morning," he recalled. "And somebody from my property in England said, 'Look, I've got some bad news. It seems as though your sister and Dodi Fayed have been in a car crash in Paris.'"
WATCH: Remembering Princess Diana
He continued: "So I went downstairs and turned on the television and was following, in fact, on CNN, and watching the broadcast, and they were quite adamant at the time that she had been seen walking away from the car and was obviously hurt, but fine.
"And then one of my other sisters called me a little later while I was watching this unfold on television, and she said, 'Look, I'm afraid it's bad news. We think she may even have brain damage.' And that was obviously a huge blow.
"And then I called my other sister, my middle sister, who was actually married to somebody who was working for the Queen, and I said, 'Look, what's going on?' And she said, 'Well, he's on the other line, right now.' And then she stopped talking, and I'll always remember hearing my brother-in-law Robert say 'Oh, no,' and then my sister Jane said, 'I'm afraid that's it. I'm afraid she's dead.'"
Charles Spencer walked with nephews William and Harry in Diana's funeral procession
Her funeral took place on 6 September 1997 at Westminster Abbey, attended by members of the royal family and a number of high-profile celebrities.
Charles and his sister Diana pictured at their childhood home in 1968
Her brother Charles delivered a moving eulogy at the service, while thousands lined the streets to pay their respects.
Princess Diana's final resting place is in the grounds of Althorp Park in Northampton.
Charles has been custodian of the estate since the death of his father, John Spencer, in 1992, and he regularly shares updates about its beautiful grounds and history.
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