LONDON -- Prince Harry has long been rumoured to be the most mischievous member of the royal family, but now we have all the proof we need.Prince Harry was "constantly in trouble at school", according to newly-discovered letters written by his mother Princess Diana.The letters were sent by Diana to former Buckingham Palace steward Cyril Dickman who worked for the royal family for over 50 years. A collection of royal memorabilia from his estate is set to be sold by auction house Cheffins on 5 January.

The letters contain intimate details of family life in the princes' early years. One letter was written by Diana to thank Dickman for a gift sent to her upon Harry's birth.

"Dear Cyril, it was so very kind of you to have sent us such a lovely card, on the arrival of our small son -- we both were greatly touched by your thoughtfulness and enormously appreciated it," reads the letter.

"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 letter continues.

"These intimate personal letters highlight a time of much joy and happiness, which was shared with Cyril," reads a statement by Cheffins emailed to Mashable.The collection also includes photos, Christmas cards and cake from the Queen's wedding.