Queen Camilla appeared to wipe away a tear during today's memorial service at St George's Chapel.
The Thanksgiving Service was being held for King Constantine of the Hellenes at Windsor Castle.
Constantine II, Head of the Royal House of Greece, reigned as the last King of the Hellenes from 1964 to 1973, and died in Athens last year at the age of 82.
King Charles, who was a second cousin of the late monarch, was unable to attend today's service while he continues to receive cancer treatment.
Queen Camilla and Princess Anne sit beside each other at St George's Chapel
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Queen Camilla looked elegant in a navy pinstripe suit
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Instead, the Queen, 76, led the Royal Family as she arrived alongside Queen Anne-Marie, the Greek King's widow.
Inside the chapel, Queen Camilla, who was sat beside Princess Anne, appeared to wipe a tear away during the service.
The King's wife looked elegant in a navy blue pinstripe blazer and skirt and a wide-brimmed hat for the ceremony.
She also accessorised with two silver leaf brooches and pearl earrings.
Princess Anne sat beside the Queen amid the King's absence
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Beatrice, Edo and Fergie also attended today's service
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Prince William, 41, announced just before today's service that he was unable to attend.
The prince pulled out of attending due to a personal matter, Kensington Palace confirmed.
GB News understands that Kate, Princess of Wales, who is recovering from abdominal surgery, is be doing well.
The Prince of Wales called the Greek Royal Family to let them know he was unable to attend.
Mike Tindall and Zara Tindall joined the royals following their holiday in Iceland
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The Greek Royal Family attended the service today
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The royals that did attend today's service include Princess Beatrice, Edoardo Mapelli Mozzi, Sarah Ferguson and Prince Andrew.
Princess Anne, Timothy Laurence, Zara Tindall and Mike Tindall also made themselves present.
The Princess of Wales did not attend as she continues to recuperate at Adelaide Cottage following her operation last month.