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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Their relief of Scottish banks, estate agents and other businesses this morning is obvious after the country voted to stay in the UK.
The referendum story is rapidly heading south. Scottish voters were promised more power - now English and Welsh voters ask "what about us?"
The Scottish independence referendum was threatening to turn into a huge boost for separatist movements in many countries in Europe.