Diana was tragically killed in a Paris car crash on August 31 1997. A month earlier she had dinner with Vogue editor-in-chief Anna Wintour and Tina Brown, then editor of The New Yorker.
The three Britons ate at the luxurious Four Seasons Restaurant in New York.
Ms Brown, who later wrote ‘The Diana Chronicles’, recounted the meal to the Daily Telegraph.
She said: “At the end of Diana’s life, she and Charles were on the best terms they’d been for a very long time.
“Charles got into the habit of dropping in on her at Kensington Palace and they would have tea and a sort of rueful exchange.
“They even had some laughs together.
“It was definitely calming down, the boys were older. They talked about their philanthropies. And she had accepted Camilla.
“One thing she had finally done was really understand that Camilla was the love of his life, and there was just nothing she could do about it.
“But she said to me at that lunch that she would go back to Charles in a heartbeat if he wanted her.”
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In 1996 the couple divorced causing a global media sensation.
Ms Brown claimed Diana had admitted to being lonely after the divorce.
She said: “Diana was desperately lonely.
“She still wished that her marriage could have survived.
“She didn’t say, ‘I’m so happy to be divorced’, she said, ‘I think we would have made a great team’.”
According to Ms Brown Diana had been planning to continue her work as a philanthropist.
She also reportedly held discussions about producing a documentary series and spoke with Tony Blair, then Prime Minister, about representing the UK in some role.
Ms Brown exclaimed: “She was a woman very much on the cusp of trying to reinvent herself in a serious way. People change.
“She was a child when she got married but at this point she knew what she wanted.
“What she realised was, with her celebrity she could actually be a seriously impactful person.”