Sarah was due to speak at a virtual event for Waterstones book store on 11 April with award-winning author Beatrice Blue.
The two were set to read from Beatrice's new book, Once upon a Mermaid's Tail and perform a draw-along with young viewers.
WATCH: Prince Philip's best family moments
"We are writing to let you know that due to unforeseen circumstances sadly Sunday's event must now be postponed. We apologise for any confusion or disappointment this may cause," Waterstones shared with fans on 9 April.
"We will endeavour to set a new date for the event, and will email you as soon as this is confirmed."
The Duke was the Duchess's father-in-law when she was married to Prince Andrew; the pair divorced in 1996.
Sarah with her two daughters, Philiip's granddaughters
The decision to postpone comes as the nation may go into an period of mourning for the passing for Philip, who died peacefully at Windsor Castle on 9 April.
A statement confirming his passing was released at noon, it read: "It is with deep sorrow that Her Majesty The Queen has announced the death of her beloved husband, His Royal Highness The Prince Philip, Duke of Edinburgh.
"His Royal Highness passed away peacefully this morning at Windsor Castle."
Clarence House shared a black and white photo of Prince Philip on their social media accounts, alongside the statement.
Per tradition, a note was momentarily hung on the gates of Buckingham Palace
As is tradition, a sign was hung on the gates of Buckingham Palace, to announce the sad news of the Duke's passing. Due to coronavirus restrictions, it was only present for a short amount of time, to prevent crowds from gathering.
The couple celebrated their 73rd wedding anniversary in November 2020
Well-wishers and royal fans however were quick to start leaving floral tributes to the much-loved family member.
The Duke's funeral will not be a state funeral and the public will not be allowed to attend.
The late royal will lie in rest at Windsor Castle, in line with his wishes, before a ceremony at St George’s Chapel.