King Charles reign won't last long, claims a royal historian

King Charles III knows his time on the throne may be short and he's a 'caretaker' of it for his son Prince William, according to a royal expert.

Royal historian Marlene Koenig says Charles - the oldest person to become king in British history- is like a "caretaker" of the throne for the future.

The Britain's new monarch, 73, knows his reign will last a fraction of the time of Queen Elizabeth II's but will carry out his duty to ensure the throne is there for future generations.

Charles was only three-year old when Queen Elizabeth II ascended to the throne in 1952, making him her heir apparent throughout her 70-year reign.

But given his age, Koenig, said: "He knows that his time is shorter," Koenig said, comparing Charles' reign to his mother's. "But he's gonna do the best that he can."

"He is and he knows he is a caretaker for the next generation," she added, referencing Charles' son and grandson, Prince William and Prince George, are both first and second in line to the throne respectively.