Queen Elizabeth II is celebrating her 93rd birthday, which this year coincides with Easter Sunday.

The queen is marking Easter by attending a service with other senior royals at St. George's Chapel on the grounds of Windsor Castle, west of London.

She was joined by Prince William and his wife Kate, the Duchess of Cambridge, and Prince Harry, whose wife Meghan, the Duchess of Sussex, is expecting their first child in the coming weeks and did not attend.

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Britain's Queen Elizabeth II waves to the public as she leaves after attending the Easter Mattins Service at St. George's Chapel, at Windsor Castle in England Sunday, April 21, 2019. (AP Photo/Kirsty Wigglesworth, pool) (AP)

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Harry and Meghan did post a birthday greeting for the queen on their Instagram page: "Happy Birthday Your Majesty, Ma'am, Granny. Wishing you the most wonderful day! Harry & Meghan."

The queen's birthday is marked this year by an unusual sunny spell that has sent flowers blooming on the extensive castle grounds.

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Sunday is the first of two birthday celebrations each year for the queen. She marks her official birthday in June with the Trooping the Color parade.

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The two-birthday tradition dates back more than 250 years, when it was instituted by King George II..

Elizabeth is the longest-reigning monarch in British history. She still maintains an active schedule, although she does not travel as often as she used to.

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Her 97-year-old husband, Prince Philip, has retired from public duties and did not attend the service.