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 outspoken Birmingham MP tweeted a picture of locksmiths ramping up security at her home after facing fresh intimidation on social media.
Islamic State's Amaq news agency released footage showing French attacker Abdel-Malik Nabir Petitjean urging Muslims to destroy the country.
Brighter skies developing across northern Britain and Northern Ireland with some sunshine tomorrow but feeling cooler again.