Queen Elizabeth II's final moments in Balmoral were "very peaceful" and without pain, a memo by her private secretary reportedly says.
Sir Edward Young wrote she "slipped away" in her sleep when she died and "wouldn't have been aware of anything".
The document is part of a new biography of the King written by the Daily Mail's royal writer Robert Hardman.
Buckingham Palace said it would not be commenting on the "unauthorised" biography.
The book, being serialised in the Daily Mail, reports the King was picking mushrooms nearby when his mother died.
According to the newspaper, the full text of Sir Edward's memo about the Queen's death on 8 September 2022 reads: "Very peaceful. In her sleep. Slipped away. Old age. She wouldn't have been aware of anything. No pain."
The document is understood to be in the Royal Archives and forms part of Mr Hardman's new book titled Charles III: New King, New Court. The Inside Story.
The biography claims a locked red box of paperwork containing two sealed letters was found at the late-Queen's deathbed - one addressed to then-Prince Charles and another to Sir Edward.
It was brought to the Royal aide shortly after he wrote the memo and also contained her choice of people to be awarded the Order of Merit for "exceptionally meritorious service" across the Commonwealth, the Daily Mail said.
The King and Queen Camilla spent an hour with her privately before she died and Princess Anne, a minister from a nearby church and the late Queen's senior dresser alternated at her bedside, the paper reports.
On the King's actions in those final hours, the Daily Mail reports that he called both of his sons and told them to travel up to Scotland as soon as possible to say their goodbyes.
The King, who had gone out to gather mushrooms and clear his head after seeing his mother, was told she had died as he was driving back to Balmoral, according to the Daily Mail.
He pulled over and was addressed for the first time as "Your Majesty", the biography says.
Afterwards when the King called Prince William via the Buckingham Palace switchboard to break the news, he told the phone operator on the other end of the line "it's me", after realising he could not reveal he had become monarch yet.
The King did try to contact his younger son to tell him personally as well, but Prince Harry was already flying to Scotland and he could not get through, the book reports.
