Diana's biographer, Andrew Morton, told to the Mail Online that she had recorded her thoughts for him on the condition of secrecy for his bombshell 1992 book, Diana: Her True Story.
Now, as the 20 year anniversary of Princess Diana's tragic car crash death in Paris looms, Morton has let the Mail publish transcripts of his interviews with her.
Diana revealed in the secret tapes how her bulimia struggle started a week after she agreed to marry Charles when he grabbed her waist and told her she was a "bit chubby" there.
The soon-to-be Princess said the cruel comment hit her so hard she began to lose weight through bulimia, which involves throwing up food.
Diana spoke in brutal detail about how she found being sick cathartic and that she "shrunk to nothing" in the months leading up to her famed royal wedding to Charles in 1981.
Diana said, "I remember the first time I made myself sick. I was so thrilled because I thought this was the release of tension.
"The first time I was measured for my wedding dress, I was 29 inches around the waist. The day I got married, I was 23-and-a-half inches. I had shrunk into nothing from February to July."
For Morton's book, Diana said she first met Charles, then 29, when she was 16 years old. At that time, he was dating her older sister Sarah, who was 22 at the time.
She described herself then as being "podgy," the British word for chubby--and being loud, which she thought Charles liked.
After several meetings, Diana told Morton on tape, she and Charles spoke at his 30th birthday party where "he was all over me."
Diana told him he looked lonely at his great-uncle's funeral – he had been killed by an IRA bomb in August 1979.
"The next minute, he leapt on me practically, and I thought this was very strange, too, and I wasn't quite sure how to cope with all this. Frigid wasn't the word. Big F, when it comes to that," said Diana, who was a virgin when she married Charles.
Diana added to Morton in transcripts, now unmasked, that she would visit him to Balmoral: "Charles used to ring me up and say: 'Would you like to come for a walk, come for a barbecue?'
"So I said: 'Yes, please.' I thought this was all wonderful."
Charles proposed to Diana in February 1981 when she was just 19.
And as the world knows, it was no fairy tale.
Prince Charles publicly admitted he'd gone back to longtime mistress Camilla Parker Bowles while still married to beautiful Diana.
Morton's stunning book Diana: Her True Story, pulled the lid off the royal marriage problems in 1992.
Charles and Di were officially divorced in 1996. People around the world were devastated when Diana died in a car accident the following year in Paris, in August 1997.
In 2005, the Prince finally married Camilla.
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