Prince William has pulled out of a public event at Windsor Castle due to a personal matter, Kensington Palace said.
The Prince of Wales was due to attend the memorial service on Tuesday for his godfather, the late King Constantine of Greece.
The thanksgiving service was held at St George's Chapel and attended by Queen Camilla along with Prince Andrew, Sarah Ferguson and the Tindalls.
King Constantine of the Hellenes was the last king of Greece and reigned from 1964 to 1974, before he was toppled in a military coup. Constantine's father was the cousin of Prince Philip, Prince William's grandfather and the husband of Queen Elizabeth II.
Constantine died in January 2023. Princess Anne and her husband Sir Timothy Laurence attended the funeral in Athens, which was held a week after his death, on behalf of the British royal family.
Prince William was due to do a reading at today's memorial service.
The palace did not give a specific reason for the Prince of Wales' sudden withdrawal, however both his father King Charles and wife Kate have been unwell recently.
BBC royal correspondent Sean Coughlan said the message from his sources was "don't panic" and Sky UK royal correspondent Rhiannon Mills said the prince's team was doing all it could to stop speculation.
"We're reassured there is no need for alarm and that the Princess of Wales continues to recover well from her abdominal surgery," Mills wrote.
"But with the intense global interest in this family in some ways they will always face a losing battle when it comes to calming the chatter about what is going on behind palace walls."
In early February King Charles was diagnosed with cancer, after a hospital treatment for benign prostate enlargement.
Just a few weeks earlier the Princess of Wales was hospitalised for a planned abdominal surgery, and spent about two weeks in hospital.
Charles has had ongoing treatment and has stepped back from public-facing duties.
He has shared public messages of thanks at being inundated with letters from well-wishers and last week was pictured smiling alongside piles of cards.
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Specific details of Kate's surgery have not been shared. Palace spokespeople have said the princess understood the interest the news would generate but she wanted to retain some privacy.
"She hopes that the public will understand her desire to maintain as much normality for her children as possible; and her wish that her personal medical information remains private," a statement said.
