Prince William unexpectedly pulled out of attending the memorial service for the late former King Constantine of Greece, where he had been due to give a reading.
Kensington Palace said this was because of a “personal matter”.
Queen Camilla led members of the Royal Family at the service at St George’s Chapel in Windsor Castle.
The King, who is being treated for cancer, was also unable to attend the service.
There are no further details of the “personal matter” that kept the Prince of Wales from being there on Tuesday morning, but royal sources have indicated there is no cause for alarm.
Prince William’s wife Catherine, who is recovering from abdominal surgery carried out last month, is continuing to do well, says Kensington Palace.
The Prince of Wales had called the Greek royal family to say he would not be able to join them.
With this year’s succession of royal medical stories, including Catherine’s operation, and the King’s prostate procedure and then cancer diagnosis, there has been much public interest in their health.
But the mood music from the palace has been one of reassurance and not to read too much into Prince William’s sudden withdrawal.
The service remembered the former King Constantine, William’s godfather, who died last year. His eldest son, Crown Prince Pavlos, gave the reading instead of Prince William.
The Queen was driven from Windsor Castle to the 15th-century chapel and greeted by the Dean of Windsor, the Right Reverend Christopher Cocksworth, at the Galilee Porch.
Princess Anne and Prince Andrew were among the royals at the memorial service, along with members of overseas royal families who had travelled to Windsor to attend.
King Charles was close to his second cousin Constantine, who was a first cousin once removed and sailing partner of the late Duke of Edinburgh.
Constantine died at the age of 82 in January last year, decades after being toppled from the throne in a military coup.