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.
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Proud parent Zara Tindall is the Queen’s granddaughter.
Former minister Norman Lamb made the accusation as MPs debated a review into the medicinal use of the drug.
The new addition to the family is the Queen’s seventh great-grandchild.