The wedding was for Lady Gabriella Windsor and her new husband Thomas Kingston.
Pippa attended today's ceremony accompanied by her husband James Matthews, parents Carole and Michael and brother James. The Duke of Sussex, wearing a three-piece morning suit and blue tie, was a surprise guest when he arrived with Lady Frederick Windsor - also known as TV star Sophie Winkleman of Two and a Half Men.
The PR director bride, who Wednesday financier Thomas Kingston, is the daughter of Prince and Princess Michael of Kent, the former of whom is Queen Elizabeth II's first cousin.
Foreign royalty included King Constantine II of Greece and his wife Queen Anne-Marie. Prince Michael is the Queen's cousin.
It was king George who changed the family name from Saxe-Coburg and Gotha to Windsor - after the castle - in 1917 due to anti-German sentiment during World War I.
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Pageboys wore cream shirts and knickerbockers for the midday ceremony conducted by Dean of Windsor the Right Rev David Conner.
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At one point, one of the bridesmaids lay on the floor outside the 15th century chapel, with another losing her floral headband.
The royal, who became a dad on May 6, looked dapper in a black tuxedo and pinstriped pants.
The couple, who had been dating for a number of years, announced their engagement in September last year after the groom proposed while they were on the island of Sark last summer.
The reception was being held at nearby Frogmore House, where Harry and Meghan staged their wedding dinner.