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 father of the drowned Aylan Kurdi said he tried to save his family as they clung to the upturned boat in the Mediterranean.
There have been various claims and counter claims about the number of Syrian people Britain has handed asylum to since the civil war.