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
Parts of the UK will feel colder than the Arctic Circle in the coming week as bitter winds make it feel -15C in some places.
At least three people have been killed in clashes in eastern Ghouta just hours after the UN Security Council backed a 30-day ceasefire.
Very Very Cold !