Prince Harry developed his rebellious streak by the time he was eight, his mother the Princess of Wales revealed in previously unseen letters.
The young prince was "constantly in trouble" at boarding school, Diana disclosed in a 1993 letter to a confidante, whom she had also told of Prince William's love for his younger brother.
In letters to the late Cyril Dickman, who served as a steward at Buckingham Palace for more than 50 years, Diana wrote that William "could not stop kissing" Harry after he was born in September 1984.
One particularly touching letter to Dickman, dated March 2, 1985, reads: "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!"
Also in the letter, Diana reveals how she and Prince Charles had been "overwhelmed" by the reaction to Harry's birth and could "hardly breathe from the mass of flowers".
Another letter, dated July 18, 1993, in a series that have emerged at auction, reveals the naughty side of Prince Harry, then aged eight.
Diana wrote to Dickman: "The boys are well and enjoying boarding school a lot, although Harry is constantly in trouble!"
Prince Harry was a pupil at private Ludgrove School in Berkshire at the time and the letter shows he had a rebellious streak at a young age.
Nine years later aged 17 he was caught smoking cannabis in the grounds of Highgrove. He came under fire again in 2005 for wearing a Nazi uniform at a fancy dress party.
Dickman started his career at Buckingham Palace watching out for fires on the roof during the Blitz.
The high esteem in which he was held by Princess Diana is reflected by the fact she confided in him about the death of her grandmother, the Baroness Fermoy, in July 1993.
In a letter dated July 18, 1993, she writes: "Thank you so much for your lovely letter concerning my Grandmother.
"Her death came as a great shock, but it has meant an enormous amount to receive messages of support such as yours. I often think of you and Mrs Dickman and hope that life is treating you kindly."
Some of the other items in the auction include signed Christmas cards from Prince Charles, Queen Elizabeth and Prince Philip.
Dickman retired at 65 and was made a Freeman of the City of London. He died aged 85 in 2012 and the letters were uncovered by his grandson who was going through his belongings.
He has decided to put them up for auction and they are valued at £15,000.
Dylan Mander, specialist at Cambridge-based auctioneers Cheffins which is auctioning off Dickman's letters, said: "It is incredibly rare to have letters from Princess Diana with such a personal touch to them.
"She would not have had friends in the sense we do so you could say Cyril Dickman was one of her closest friends."
Cheffins' sale takes place in Cambridge on January 5.
Originally published in Telegraph UK.