Kate, who is more than eight months pregnant, will stay at home, but William and Harry are expected to be among the congregation to watch Thomas van Straubenzee marry Lady Melissa Percy.
The Duchess' final official public appearance was last weekend when she attended the Trooping the Colour ceremony with other members of the Royal Family.
More top news
The world's populations of animals have dropped by more than half since the 1970s, according to a new survey by the WWF and the ZSL.
Did the International Space Station take a wrong turn?
The wife of a British aid worker held by Islamic State militants has recorded a message to his captors appealing for his life to be spared.