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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The Government scheme would benefit those aged under 40 but will only run for five years.
Joseph Rowntree Charitable Trust and the Roddick Foundation are under investigation by the watchdog over grants made to Cage.
The Duke of Cambridge became a fearsome samurai warlord - at least temporarily - when he visited the set of a long-running Japanese drama.