Diana, Princess of Wales recorded an account of her struggle with bulimia and anxiety over her relationship with Charles, it has been reported.
Her struggle with the disorder began just days after the Prince of Wales proposed in February 1981, according to a transcript of tapes published by the Daily Mail.
In the excerpts she described how, finding being sick cathartic, she "shrunk to nothing" in the months between their engagement and wedding.
She 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."
Then 19-year-old Lady Diana Spencer, she described feeling isolated and intimidated as a royal-in-waiting.
"The bulimia started the week after we got engaged," Diana said "My husband put his hand on my waistline and said: 'Oh, a bit chubby here, aren't we?' and that triggered off something in me."
Diana reportedly recorded the account in 1991 as their marriage began to disintegrate.
Twenty years on from their mother's death, Princes William and Harry have spoken publicly about their own experience of mental health.
As part of their Heads Together campaign, Harry told how he sought counselling to deal with his grief.
William said it took him "almost 20 years" to be able to speak about his mother openly and "more honestly".