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.
More top news
A woman has appealed to find the "dapper" stranger who confronted the man who allegedly groped her on a bus.
Questions on Russia and Syria are sure to dominate Defence Secretary Michael Fallon's meeting with his US counterpart Ash Carter.
Smoking could kill two million Chinese in 2030 with the country facing a "growing epidemic of premature death from tobacco".