Britain's Queen Elizabeth, 92, is feeling unwell and will not attend a special service on Thursday at St Paul's Cathedral, Buckingham Palace said.
"The queen is feeling under the weather today and has decided not to attend this morning's service ... marking the 200th anniversary of the Order of St Michael and St George," a palace statement said.
"Her Majesty will be represented by The Duke of Kent as Grand Master of the Order," it added.
Elizabeth, the world's oldest and longest-reigning living monarch, celebrated her 92nd birthday in April.
The British monarch underwent surgery to treat an eye cataract last month, though the queen is known for her robust health and still carries out hundreds of engagements every year.
In 2016, she missed the annual Christmas service she usually attends with her family on her estate in Sandringham, eastern England, due to a heavy cold.
Elizabeth was born on April 21, 1926, and became queen in 1952 at the age of 25, meaning she has now reigned for more than 66 years.
While some members of the royal family have been criticised for leading lives of luxury, Elizabeth is widely admired in Britain for her quiet devotion to duty over many decades.
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