Royal author reveals he knows MAJOR secret about how The Queen died - 'It CAN'T come out'

However, speaking to The Camilla Tominey Show about the day he said "there are other aspects of that day that I haven't reported on."

The book, being serialized in the Daily Mail, is called Charles III: New King, New Court, The Inside Story.

He said: " I wanted to do it as sensitively as possible. You know, there are other aspects of that day that I haven't reported on.

"I don't know the full extent and I don't think it's appropriate that it should come out."

​The book also reveals how, shortly after Sir Edward wrote his note, a footman brought a locked red box of paperwork found at the late Queen’s deathbed which contained a sealed letter to her son.

Hardman told GBNews: "The other thing that I absolutely that put the hairs on the back of my neck when I heard about it was that her last red box.

"She'd had a red box brought to her every single day of her life.

"As Queen, she was only ever allowed two days off, her birthday and Christmas Day.

"Other than that the red box appeared every day full of papers and stuff to be dealt with. And right at the they opened up her last red box."

Speaking about Charles' letter he said: "We'll probably never know what was in it and I understand that.

"I think there was certainly a sense in those last few days she wanted to get the country back on its feet.

"That's why she was so keen to be on her feet and looking the part for that last photograph.

"All summer there hadn't really been a government to speak of, she wanted to appoint Liz Trust and get that done.

"Obviously that involved accepting the resignation of Boris Johnson.

"She was very buoyed up. Having done that. That was a weight off her mind."