The Prince of Wales, 41, had been expected to attend the remembrance event at Windsor Castle on Tuesday.
The source would not elaborate further on the last-minute change of plans, but added that William’s wife, the Princess of Wales – who is recuperating after abdominal surgery in January – “continues to be doing well.”
Kate has been out of the public eye as she continues to recover at home and is not expected to return to royal duties until after Easter.
King Charles III is also absent from the service as he continues his treatment for an undisclosed form of cancer. The 75-year-old monarch recently revealed he had been reduced to tears by many messages of support he has received since his cancer diagnosis.
Queen Camilla is leading the family for the gathering in honor of the last Greek monarch at St. George’s Chapel.
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Constantine II of Greece, second cousin to King Charles III, died in January last year at the age of 82.
Charles and Constantine enjoyed a close relationship, with the British royal even naming his cousin as godfather to his firstborn son, William.
After a military coup in 1967, Constantine and his family fled to Rome, and the military regime appointed a regent to take his place.
The Greek monarchy was abolished on June 1, 1973, when the military regime proclaimed the country a republic – a decision that was backed by a subsequent referendum. Constantine accepted the abolition after another referendum was conducted by an elected civilian government in 1974.
