Britain's Prince William, whose wife Catherine is recovering from surgery and father King Charles III is undergoing cancer treatment, on Tuesday pulled out of attending a memorial service, citing a "personal matter".
William's Kensington Palace office declined to elaborate but said Catherine, the Princess of Wales, continued to be "doing well".
William had been due to attend the memorial at Windsor Castle, west of London, for the late King Constantine of Greece, who was also one of his godfathers.
The Prince of Wales, who was to have delivered a reading, had called the Greek royal family to let them know he was unable to attend, the palace said.
This year has seen a sudden shortage of frontline royals, after the almost simultaneous hospitalisations of the king and Catherine earlier this year.
Charles was admitted for surgery for a benign prostate condition last month but subsequently diagnosed with an unrelated cancer.
He has withdrawn from public-facing duties while he undergoes treatment.
Catherine underwent abdominal surgery at the same time and is expected to be out of action until at least the end of March, following a two-week hospital stay.
In the absence of Charles, the king's wife Queen Camilla led members of the royal family at Tuesday's memorial service at Windsor's Saint George's Chapel.
Dozens of foreign royals including Spain's King Felipe VI and Queen Letizia also attended.
Charles, who has been staying at Windsor Castle, was not expected to meet them privately.
King Constantine of Greece, Charles's second cousin, died aged 82 in January 2023.
The last member of a century-long dynasty, Constantine had reigned for just three years when the army seized control of the country in 1967.
Nearly eight months after the junta seized power, Constantine organised a military counter-coup that failed.
He fled to Rome with the rest of the royal family, and later to London.
The junta abolished the monarchy in 1973 and Greeks voted not to restore the royal family after the restoration of democracy in 1974.
The British Crown was represented by Princess Anne, King Charles III's sister, at Constantine's funeral in Greece last year.
