Tom will take up the role in St George's Chapel in support of his brother as he marries Princess Eugenie in front of a worldwide audience.
Even though Camilla Parker-Bowles was front and center at both Prince William and Kate Middleton's wedding in 2011 and Prince Harry and Meghan Markle's wedding back in May, Prince Charles' wife won't be present to see Princess Eugenie marry her fiancé, Jack Brooksbank.
The young royals will have starring roles as page boy and bridesmaid.
The statement from Buckingham Palace said Eugenie and Jack were "delighted" that Cabot was involved in the celebration of their nuptials. Princess Eugenie is godmother to Maud.
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While Prince Edward and Sophie, Countess of Wessex's children Lady Louise, 14 and Viscount Severn, 10, will serve as Special Attendants.
With Princess Eugenie's wedding just around the corner and more details emerging about the event, it's been revealed that Camilla, Duchess of Cornwall (71), won't be in attendance because of a prior engagement, reports Harper's Bazaar.
It has already been announced that 1,200 members of the public have been invited into the grounds of Windsor Castle for the wedding.
A palace spokesperson said: "The attendees will view the arrival of the congregation and Members of the Royal Family, listen to a live broadcast of the Marriage Service and watch as the Bride and Groom depart St George's Chapel at the end of the ceremony".
The couple get married on Friday in Windsor, Berkshire.