The late Queen is protecting her right-hand woman from "beyond the grave" after handing her a trump card, a friend has claimed.
Angela Kelly, the former monarch's dresser and confidante, famously published two books about her work after being granted extraordinary permission from her boss to do so.
It was understood she had the late Queen's blessing to pen a third book, although recent reports have suggested King Charles scuppered those plans after getting Ms Kelly to sign an NDA (non-disclosure agreement) in exchange for a new grace-and-favour home in Yorkshire.
It is claimed the agreement banned Ms Kelly from using words such as 'Palace' and 'King'.
However, it is now being reported that Ms Kelly has told friends she is holding a trump card over the King - who is said not to be a fan of hers - with the late Queen reportedly having given her a letter granting her express approval for three books.
And a source now says the letter is a "formidable weapon" for Ms Kelly, who is being protected by her former boss from "beyond the grave".
The source told the Mail On Sunday : "It's certainly a formidable weapon in her arsenal to have a letter like that from Queen Elizabeth II in her possession. It's quite something because it seems she is protecting her favoured assistant from beyond the grave.
"It is pretty important because it includes the Queen's wishes, and no one would want to ignore those."
After the death of the late Queen last September, Ms Kelly, who had worked for the monarch for 30 years, was left facing an uncertain future - especially as Queen Camilla had her own staff looking after her wardrobe.
It was believed she was going to be allowed to stay at her grace-and-favour home in Windsor, however, last month, she appeared to confirm this was not the case.
She took to Instagram to say she would be leaving a property on the Windsor estate and also seemed to confirm that her work phone had been disconnected.
In recent days, Ms Kelly, who is originally from Liverpool, was spotted emotionally packing up her home in order to move north to be closer to her family.
However, as pictures emerged of Ms Kelly packing her belongings from her home in Windsor into a removals lorry, she took to her Instagram stories to share videos with several cryptic quotes.
In one story, she shared the lyrics of a song called Broken Angel, which features the line: "You are the one. I miss you so much. Now you're gone."
While in another story, a quote said: "No matter how kind and generous you are, you'll never satisfy an ungrateful person. Learning how to remain calm when you're disrespected is a superpower."
It comes after reports the King felt that an updated version of Ms Kelly's book, The Other Side Of The Coin, which was released last year, went "too far" in terms of revealing intimate details about the Royal Family.
It detailed what the late Queen did after the funeral of her beloved husband Prince Philip in April 2021.
"I helped her off with her coat and hat and no words were spoken. The Queen then walked to her sitting room, closed the door behind her, and she was alone with her own thoughts," Ms Kelly wrote.
A source previously told the Daily Mail: "His concerns were solely about protecting both the dignity of the Queen and the Crown.
"Nothing has changed with his mother's death. If anything, he is more determined to defend her reputation and her legacy."