If Queen Elizabeth's biography was to be printed right now "it would probably crush the rest of King Charles' reign", said a formerroyal butler Paul Burrell.
According to express.co.uk, he said the late monarch's legacy would also overshadow Charles' reign.
Charles became the king after his mother, Queen Elizabeth, died at the age of 96 in September last year.
Elaborating on his remarks, Burrell said 70 years on the throne is "an impossible act to follow" and something the King is well aware of.
"I think that would be dynamite right now, I think her legacy is still incredibly strong so I would crush the King’s reign because she is an impossible act to follow.
"There is much that can be said in the Queen’s own words after her passing that would upset the King and Queen."
The former royal butler suggested Queen Elizabeth's biography may be written by William Shawcross who wrote the Queen Mother’s biography or "it may be written by a woman."
But he stressed: "You can bet if it is written it will be edited and redacted by the King so we may never know the real truth."