Letters sent by Princess Diana to a Buckingham Palace head steward reveal that Prince Harry was "constantly in trouble" at boarding school.
Diana sent the six handwritten notes - which are expected to sell for thousands of pounds at auction - to Cyril Dickman in the 1980s and 90s.
In one of them, sent five days after Harry's birth in September 1984, Diana said it had been a "particularly happy time"
She added: "William adores his little brother and spends the entire time swamping Harry with an endless supply of hugs and kisses, hardly letting the parents near!
"The reaction to one tiny person's birth has totally overwhelmed us and I can hardly breathe for the mass of flowers that are arriving here!"
In another letter, sent in October 1992, Diana said: "The boys are well and enjoying boarding school a lot, although Harry is constantly in trouble!"
After the death of her father in the same year, Diana tells Mr Dickman: "Thank you for thinking of us at this sad and difficult time... I miss my Father dreadfully."
The collection also features a boxed piece of cake from the Queen's wedding in 1947, Christmas cards and photos.
Cheffins auctioneers in Cambridge said there had been international interest.
Auctioneer Luke MacDonald told Sky News: "They give a more personal insight into the lives of the royals (that) perhaps we can relate to far more than the more formal times we see them in the press and on television.
"Her Royal Highness Princess Diana was adored by so many people. I think people feel that to actually have a letter, handwritten by her - it's something very special."
On Mr Dickman's retirement in 1992, Princess Diana wrote: "Cyril, you will be greatly missed by this particular Lady - thank you from the bottom of my heart for being so kind to me throughout the years and take great care of yourself, fondest love from Diana x."
This article originally appeared on Sky News.