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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Drivers' union ASLEF suspended industrial action scheduled for next week but the RMT Conductors' union strike remains in place.
Dozens of African-American former agents will receive $24 million in settlement of a class-action lawsuit alleging racial discrimination.