Prince Harry attempted to cancel the publication of his memoir after visiting the UK last summer, according to a publishing source in the United States.
Spare is due for release on January 10 but was leaked after the book inadvertently went on sale early in Spain.
The Duke of Sussex's autobiography has delivered a number of bombshell claims about the royal family, from an accusation of Prince William physically assaulting his brother during a row about Meghan Markle, to Harry and William asking their father King Charles not to marry Queen Consort Camilla.
However, a source in the US claims that the revelations almost never came to light.
After Harry and Meghan attended the Queen's Platinum Jubilee in June, the team working on Spare were reportedly told: “He’s pulled it. He doesn’t want to do it.”
The duke is thought to have received a multi-million pound advance for the book but a report in The Times says insiders at publisher Penguin Random House speculated that Harry was told that if the book were published while the Queen was alive, there would be “no way back”.
A source involved in the publication of Spare said: “The book was all ready to go but visiting his grandmother he had second thoughts.
“That was such a dark day. Everyone had been working so hard on this project in utmost secrecy — and with the expectation this would turn out to be one of the biggest and most historic books we would ever get to publish.
“And then, just after Harry’s last trip to London, the boss walked in and said: ‘He’s pulled it. He doesn’t want to do it.’"
The source added that everyone involved was 'devastated.'
While Harry has clearly changed his mind in favour of publication, his reported hesitation suggests the 38-year-old had mixed feelings about the devastating impact his memoir would have.
The source also described how the book had to be made to sound “like he was actually English”, with Harry having apparently developed several American speaking traits since moving to California three years ago.
The book is set for release next week.