Sarah, Duchess of York has postponed her upcoming appearance in Perth so as not to "distract or detract" from King Charles' upcoming royal tour alongside Queen Camilla.
The 64-year-old had been due to visit Australia in October this year to attend the Festival of Fiction in her capacity as a special guest.
A statement shared on the Festival's official Instagram 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."
It continued: "Due to exceptional demand, she will be returning to Perth on November 1 at 6.30pm for a special event at Joondalup Resort and apologises to anyone who is inconvenienced by the change of date."
According to the Festival's website, the mother-of-two "will be discussing her prolific career as an author of over fifty books and telling us about her latest exciting new projects."
The palace confirmed Their Majesties' autumn royal tour of Australia and Samoa last week.
The King, 75, and Queen, 76, will visit Australia at the invitation of the Australian Government, where 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 U.K.
Regarding their visit to Australia, a Palace spokesperson said: "Their Majesties will visit the Australian Capital Territory, New South Wales and Samoa.
"The King's doctors have advised that a further extension to Their Majesties' trip should be avoided at this time, to prioritise His Majesty's continued recovery.
"As with all His Majesty's recent engagements, his programme in both countries will be subject to doctors' advice, and any necessary modifications on health grounds."
Their visit will be the first trip Down Under by a reigning monarch since 2011, when Elizabeth II visited for the last time. Charles and Camilla last visited Australia in 2018, when they represented the late Queen Elizabeth II at the Commonwealth Games.