NEWLY unearthed FBI files describe Prince Charles’s mentor, Lord Mountbatten, as “a homosexual with a perversion for young boys”.

The decorated war hero was seen as a father figure to the young Charles, his great-nephew, and is even credited with introducing his father to Queen Elizabeth, The Sun reports.

The Prince of Wales was mentored by Mountbatten as he grew up, and described him as “the grandfather I never had”.

Mountbatten - who also features in The Crown on Netflix - even coached his great-nephew on his love life.

As the last viceroy of India, Mountbatten oversaw the British withdrawal and eventually became the chief of defence until 1965.

The military man was also well-known for his sexual appetites, sparked in part by his own comment about his wife: “Edwina and I spent all our married lives getting into other people’s beds.”

More is known about Edwina’s extramarital affairs, but in the 75-year-old dossier, an aristocratic source close to them, interviewed by the FBI in the 1940s, described them as “persons of extremely low morals”.

British historian Andrew Lownie used freedom of information laws to source the files, and quotes them in his new biography of the famous couple.

Another source in the FBI files claimed that Mountbatten’s private life made him “an unfit man to direct any sort of military operations.”

In Lownie’s book, The Mountbattens: their Lives & Loves, Ron Perks, who was Mountbatten’s driver in Malta in 1948, also breaks his silence.

He tells how Mountbatten had one favoured destination, the Red House near Rabat, which “was an upmarket gay brothel used by senior naval officers”, which he had not realised at the time.

Homosexual acts were banned in the UK until 1967, and many memos about Mountbatten’s sexuality have been edited or destroyed since.

Disturbing allegations also claim he had an interest in underage boys.

Elizabeth de la Poer Beresford, Baroness Decies, when interviewed about another matter, had mentioned being an intimate of Queen Elizabeth, Queen Mary and their ladies-in-waiting.

The first FBI files date back to February 1944, soon after Mountbatten had become supreme allied commander of southeast Asia.

It reads: “She states that in these circles Lord Louis Mountbatten and his wife are considered persons of extremely low morals.

“She stated that Lord Louis Mountbatten was known to be a homosexual with a perversion for young boys.

“In Lady Decies’ opinion he is an unfit man to direct any sort of military operations because of this condition. She stated further that his wife Lady Mountbatten was considered equally erratic.”

Mr Lownie believes US spies began compiling intelligence on Mountbatten’s private life after he was made supreme allied commander of Southeast Asia during the Second World War.

It’s not known if the material was passed to the British government.

Mountabatten was assassinated in an IRA bomb attack in 1979, that also killed Nicholas, John’s mother Lady Brabourne and a 15-year-old crew member, Paul Maxwell.

The book, The Mountbattens, will be published on August 22.