King Charles knows his time on throne may be short?

King Charles III, set to be officially crowned on may 6, knows his time on the throne may be short and he's a 'caretaker' of it for his elder son Prince William, a royal expert has claimed.

The 74-year-old monarch, who is the oldest person to become king in British history, was only three-year-old when the Queen ascended to the throne in 1952, making him her heir apparent throughout her 70-year reign.

But given his age, royal historian with a focus on British and European royalty Marlene Koenig, according to Insider, has said the new monarch is acutely aware of how much time he could have left.

"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," Koenig, per insider, claimed when William's father ascended to the throne following the Queen's passing in September.

"He is and he knows he is a caretaker for the next generation," she added, referencing Prince William, his eldest son, and Prince George, his eldest grandson. William, 40, and George, 9, are both first and second in line to the throne respectively.

Doing his best in the years he has left is something Charles indicated he's acutely aware of in his first speech as king, pledging to uphold constitutional principles "throughout the remaining time God grants me."

King Charles latest move - as he has officially recognised Harry and Meghan's children Archie and Lilibet as Prince and Princess - suggests he's trying his best to overcome crisis within the family to make the way clear for the future king, prince William.