George III was renowned as a paragon of domestic rectitude, before he descended into derangement.

His court was frugal and respectable — mistresses were absent for the first time in a century — and George was determined to ensure the proper moral education of his 15 children. The King's homely virtues did not, however, rub off on his nine sons.

Previously unseen letters, to be sold at auction today by descendants of the King's personal secretary, reveal George's growing frustration as his third son, who would become William IV, ignored his advice and slid into debt and disobedience.

Prince William was sent away to join the Navy when he was only 13 years old, in no small part to get him away from his increasingly debauched older brother.

Times Archive, 1920: Obituary of George III

Over the last nine years of his late Majesty's life an awful veil has been drawn. In periods of the deepest national solicitude, his mind has felt no interest

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The future George IV, the King's oldest son, had by this time developed a reputation for a dissolute love of drink, fashion and fornication, and was soon scandalising his sober father by consorting with a procession of mistresses and opposition politicians.

George urged William not to follow the same path, writing to him when he was a midshipman in 1779: “You are now launching into a scene of life, where you may either prove an Honour, or a Disgrace to your Family; it would be very unwelcoming of the love I have for my children, if I did not at this serious moment give you advice, [on] how to conduct yourself dictated from no other motive than the anxious feelings of a Parent, that his Child may be happy, and deserve the approbation of Men of worth and integrity.”

“Though when at home a Prince, on board of the Prince George you are only a Boy learning the naval Profession; but the Prince so far accompanies you that what other Boys might do, you must not: It must never be out of your thoughts that more Obedience is necessary from You to Your superiors in the Navy, more Politeness to your Equals, and more good nature to your Inferiors than from those who have not been told that these are essential for a Gentleman.”

William soon became a capable naval officer, winning praise from Horatio Nelson, and George expresses fatherly pleasure at his son's progress: “I flatter myself your conduct has changed this year much to your advantage.”

But William had difficulty working with others, and a heated quarrel with his first lieutenant ended his chances of high command. Instead, he fell into the typically Hanoverian pattern of rebelliousness and womanising.

While commanding the frigate Andromeda in Halifax, Nova Scotia, he fathered an illegitimate son and was said by a friend to be “perfectly acquainted with every house of a certain description in the town”.

George began urging his son to be more like his other elder brother, the “perfectly compliant” Prince Frederick. When this failed to have the required effect, he lambasted his son's “unhappy disposition to resist control”. After brief hopes of repentance, the King's letters end with a bitter condemnation of his son's “love of improper company” and anger at his mounting financial problems.

Thomas Venning, a manuscript specialist at Christie's in London, where the letters will be sold, says the letters give a fascinating personal perspective on the dysfunctional 18th-century Royal Family.

“You can see George III getting more and more disapproving of his conduct, [but] you can tell it is going in one ear and out the other.”

The 22 letters from the King - best known for losing the American colonies and his years of madness - came into the possession of William's secretary, Sir John Barton, and are being sold by his descendants. The lot includes 59 letters from William's mother, Queen Charlotte, his brothers and Mrs Jordan, his partner of 20 years. They may fetch up to £20,000.

Heirs and graces

1760 George III becomes King

1762 George, Prince of Wales, is born

1765 Prince William is born

1779 William joins the Navy and correspondence with his father begins

c1785 Correspondence ends

1791 William begins living with the actress Mrs Jordan

1810 George III's dementia returns and the Prince of Wales becomes Regent

1811 William separates from Mrs Jordan

1818 William marries Princess Adelaide of Saxe-Meiningen

1820 George IV becomes King after the death of his father

1830 George IV dies and William IV begins a seven-year reign

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