William, the Prince of Wales, had been due to give a tribute in honour of Greece’s late King Constantine, on Tuesday morning, local time at St George’s Chapel in Windsor Castle.
But he pulled out of the reading leaving Queen Camilla to lead the royal party.
Kensington Palace did not elaborate on the “personal matter” requiring William’s attention.
It had been speculated that Kate was the reason for his exit from the event.
The princess has abdominal surgery in January and is not due to resume any royal duties until after Easter. She has not been seen in public since her hospital stay.
But Kensington Palace has insisted that Kate’s health has not deteriorated.
“The Princess of Wales continues to be doing well,” it said in a statement.
Although it did not elaborate further.
King Charles, 75, also missed the service in Windsor due to his cancer treatment. He was at Windsor on Tuesday morning but departed before the service started.
The King was diagnosed with “a form of cancer” after an issue of concern was spotted during an unrelated prostate operation.
Instead Prince Pavlos, King Constantine’s son, was tasked with doing the reading. William reportedly called the Greek royals to let them know he would be unable to attend.
Accompanying Camilla to the service were Princess Anne, Prince Andrew, Sarah, Duchess of York and Mike Tindall.
Dozens of royals from other nations were in attendance including Spain’s Felipe VI and Queen Letizia. The Greek royal family does not have an official position in the country following a republic being declared in 1974.
However, more recently the family have been welcomed back to Greece. Constantine II died in January last year, aged 82
Kate’s behind closed doors recovery
Princess Kate has been recuperating at Adelaide Cottage in the grounds of Windsor Castle. She has been recovering well enough to join her family on a school half tern holiday at Anmer Hall in Norfolk, close to Sandringham in England’s east.
Earlier this month, the Daily Mail cited a source that said “Catherine is recovering well”.
Kate’s absence from the public eye has sparked concern, regardless of the family’s statements talking up her recovery.
She has not been seen out and about since Christmas Day last year, when she and William took their children George, Charlotte and Louis to a church service.
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