King Charles suffers blow as Prince Harry fails to avoid clash with royal tour after Fergie's gesture

However, its October release will clash with King Charles's high-profile appearance at the Commonwealth Heads of Government Meeting in Samoa.

This is the first time he will be attending since acceding to the throne and becoming head of the global organisation.

The meeting will be held between October 21 and October 25, with Harry's paperback being published in the US on October 22 and then in the UK two days later.

Buckingham Palace announced in July the monarch planned to attend as part of an Autumn tour in Oceania, despite his ongoing cancer treatment.

Despite coinciding with the King's tour, Harry's re-release of Spare will not include any new bombshells or chapters.

This is highly unusual for an international best-selling book, as Spare topped the Guinness World Record as the fastest-selling non-fiction book of all time.

The Duke of Sussex overshadowing the King's tour comes as Sarah Ferguson, 64, recently axed an upcoming appearance in Australia to not clash with the monarch.

She had been due to visit Australia in October to attend the Festival of Fiction as a special guest.

However, a statement shared on the Festival's social media read: "With regret, Sarah, Duchess of York has decided to withdraw from the Festival of Fiction, so as not to distract or detract in any way from the tour of Australia by His Majesty the King, which has recently been announced.

"Due to exceptional demand, she will be returning to Perth on November 1st at 6.30pm for a special event at Joondalup Resort and apologises to anyone who is inconvenienced by the change of date."

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According to the Festival's website, the mother-of-two "will be discussing her prolific career as an author of over 50 books and telling us about her latest exciting new projects."

The King, 75, and Queen, 77, will visit Australia in October at the invitation of the Australian Government.

Their programme will feature engagements in the Australian Capital Territory and New South Wales.

Meanwhile, their State Visit to Samoa will celebrate the strong bilateral relationship between the Pacific Island nation and the UK.