The Prince of Wales was described as a "gruesome child" in a letter written by the Queen's cousin recalling how he took pleasure in watching her have a painful blister lanced.
Princess Alexandra wrote the letter in 1952, when the Prince of Wales was four, during one of the Royal Family's stays at Balmoral.
In the handwritten note addressed to "Darling Eddie", her brother Prince Edward, the Duke of Kent, she told how she had been playing the parlour game "stone" when a man named Martin "bit me quite accidentally on the mouth, which was extremely painful".
She went on to detail the injury, which had caused a blister that she had then bitten, making it bleed.
"So this morning Dr Middleton came and pierced it! Ho Ho. Charles was fascinated and insisted on watching. Gruesome child don't you think?", she wrote.
She then added that the Prince of Wales had been in bed for three days, having had some trouble with his ear.
The letter, written on Balmoral Castle headed paper and dated September 21, detailed the ways in which the Princess had been passing the time.
She wrote:"There have been shooting lunches most days" and mentioned meeting John Slim, a soldier and hereditary peer.
"I have met him only twice, not a new love if that's what you are thinking," she writes.
She also mentions The Duke of Edinburgh, writing: "We had a long discussion about black stockings. Too stupid. He is so sweet."
Princess Alexandra, also known as the Honourable Lady Ogilvy, then concludes with a mention of a story written about her in the tabloid newspaper The Sunday Graphic, which folded in 1960.
She wrote: "By the way did you see that ridiculous article about me in the Sunday Graphic. Apparently I am called the Bonny Maid of Deeside.
"Philip teased me as it said I was going with the Wrens to be a nurse."
She then signed off: "Well look after yourself. With very, very many hugs & XXXX from Georgie."
Princess Alexandra, who is now 73, is the youngest granddaughter of King George V and Queen Mary and was a bridesmaid at The Queen's wedding to the Duke of Edinburgh.
She was known as Princess Alexandra of Kent until she went on to marry Sir Angus Ogilvy, a businessman with whom she had two children. It is believed that Georgie was a family nickname for her.
The Duke of Kent, 74, is now perhaps best know as President of the All England Lawn Tennis and Croquet Club, and for presenting the trophies to the Wimbledon winner and runner-up.
The letter, which has been in a private collection for the past 25 years, will be auctioned off at the Radisson Edwardian Hotel at Heathrow on February 13, and is expected to fetch £400 (€455).
Auctioneer Richard Davie said: "It is a rare letter with good references to the Royal Family."