The King and Queen led a celebration of the ancient Order of the Garter but the Princess of Wales – who attended Trooping the Colour on Saturday – was absent.
Both Kate and the King have been undergoing treatment for cancer.
The Duke of York – a member of the order – did not join the service either after stepping down from public life following controversy over his friendship with convicted sex offender Jeffrey Epstein.
Ladies and Knights of the Garter walked through the grounds of Windsor Castle for the annual service commemorating the country’s oldest and most senior order of chivalry, founded in 1348 by Edward III.
In warm weather, one of the guards fell backwards at one point.
Composer Andrew Lloyd Webber was one of several new Companions of the Order installed during the service at St George’s Chapel.
Lord Lloyd Webber, whose musicals include The Phantom Of The Opera and Cats, attended a private investiture in the castle’s Garter Throne Room earlier in the day.
Also invested were the Duchess of Gloucester as a Royal Lady of the Garter.
Lord Kakkar, emeritus professor of surgery at University College London, and Air Chief Marshal the Lord Peach, were both made Knight Companions.
In line with tradition, Charles and Camilla were at the back of the procession, dressed in white plumed hats and dark blue velvet robes, as were the other members of the order, including the Prince of Wales, Princess Royal and Duke of Edinburgh.
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Hundreds of spectators lined the route.
It is the start of a busy period for the royals. The King and Queen are expected at Royal Ascot in the coming days, while they will be hosting a state visit by Japan’s Emperor the following week.