A letter written by the Queen's grandfather in 1908, in which he complains boys of the time are "too forward", will be sold at auction.
The future George V also apologises that his son Prince Albert - who would become George VI - was "rather shy".
The letter was written two years before George became king, and a day after the then 12-year-old Albert attended a Royal Navy cadet interview.
Auctioneers said the letter gave an "intimate picture" of a young Albert.
'My second son'
The note, written by the then Prince George, is addressed to Admiral Sir Wilmot Hawksworth Fawkes, president of the interview committee for entry as a cadet to the Royal Navy.
In it, he thanks Sir Wilmot for interviewing his son.
"It was very kind of you writing to tell me about my second son having been up before the interview committee yesterday of which you were the president," he says.
"The princess and I are both delighted to hear that you were pleased with the way in which he answered the questions put to him."
It continues: "I am sorry that in spite of all you did, you were unable to put him at his ease, he has always been rather shy, but I think it is better than being too forward, which many boys are in our days."
King George VI - portrayed by Colin Firth in the 2010 film, The King's Speech - became monarch following the abdication of his older brother, Edward VIII in 1936.
He was king during World War Two and succeeded by Queen Elizabeth II.
Historians have recorded that he neither desired, nor expected to become king.
He was considered to be a shy man who had a nervous stammer.
Michael Kousah, of Sworders Fine Art Auctioneers in Essex, said the letter was "remarkable" and showed how the future king was aware of his son's "shyness and sensitivity".
The letter, one of 11 addressed to Sir Wilmot by royalty, will go under the hammer on 26 September.
The collection is expected to sell for £800 to £1,200.