Royal experts believe that Prince Harry's upcoming memoir will be the last straw in his deteriorating relations with the British royal family.

Royal editor Richard Kay wrote for The Daily Mail that for a whopping deal of $20million, the Duke of Sussex is bound to reveal a number of inside details about the royal family. He said, "In order to justify the vast sums that are being bandied around, it can only be one thing: the book will be a tell-all. He must therefore be prepared to expand on all the explosive 'truth bombs' he and Meghan detonated under the Royal Family with Oprah back in March."

"Might he name the family member who, according to the couple, made comments about the colour of their then unborn son Archie's skin?....Will he name the anonymous member of staff who appeared to airily dismiss Meghan's worries about her mental welfare?" Kay added.

The royal expert also noted that Buckingham Palace's biggest fear at the moment is who will be in the firing line in the book. Insiders fear the targets could include Prince Charles, Prince William, Duchess Camilla, and the institution itself. A courtier explained, "He is unlikely to criticise his late mother or his grandmother the Queen. But for everyone else it's tin-hat time."

Meanwhile, royal expert Dan Wootton claimed that Harry's revelations could potentially threaten his father Prince Charles's plan to make his wife Camilla his queen. The palace has previously said that the Duchess of Cornwall will be called the Princess Consort when her husband takes over the throne, but the Prince of Wales reportedly wants to name her Queen Consort.

A source explained, "Let's be honest, Harry has never been close to the Duchess of Cornwall. If he documents their fraught relationship in the book it could be very damaging at a time when Charles is laying the groundwork for her to become Queen."

Wootton also noted that the memoir will overshadow the Platinum Jubilee which will be celebrated on a large scale next year to mark 70 years of Queen Elizabeth II's reign. A senior royal source said about the book, "This is the no going back moment – the final nail in the coffin of the Royal Family's relationship with Harry. The emotional turmoil as they wait over a year for publication is going to be torturous."

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Princess Beatrice, Princess Eugenie, Princess Anne, Princess Royal, Prince Andrew, Duke of York, Prince William, Duke of Cambridge, Catherine, Duchess of Cambridge, Meghan Markle and Prince Harry attend Christmas Day Church service at Church of St Mary Magdalene on 25 December 2017 in King's Lynn, England. Chris Jackson/Getty Images