William had been due to attend a service for the late King Constantine of Greece at Windsor Castle on Tuesday.

But less than an hour before the St George's Chapel memorial was due to start, it was announced that William would not be present.

Kensington Palace would not elaborate further but said the Princess of Wales, who is recovering from abdominal surgery, continues to be doing well.

The Prince of Wales called the Greek royal family to let them know he was unable to attend.

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The late King Constantine of Greece, who died in January last year, was William's godfather and a first cousin once removed of the King's father, the late Duke of Edinburgh.

Charles, who is being treated for cancer, also missed the service and the Queen led members of the Royal Family in attendance.

Dozens of royals travelled to Windsor for the memorial but Charles - who has been staying at the castle - is not expected to meet them privately.

He is instead due to leave the Berkshire residence at some point on Tuesday.

The Duke of York could be seen making his way into the service alongside family members including his ex-wife, Sarah, Duchess of York, their daughter Princess Beatrice, and Zara and Mike Tindall.

The Queen was the final British royal to arrive.

The family was joined by the late Constantine's widow, Queen Anne-Marie, their eldest son, Crown Prince Pavlos, and his wife, Crown Princess Marie Chantal, as well as other Greek and Danish royals.

The service saw Pavlos step in to give a reading that William had been due to deliver.

Charles, 75, was diagnosed with an undisclosed form of cancer just over a fortnight ago.

The King is not carrying out public duties or attending engagements during his ongoing treatment.

Meanwhile, Kate underwent abdominal surgery in January and remained in hospital for two weeks before returning to the Wales's Windsor home to recover.

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The planned surgery was understood to be routine and the princess's condition was non-cancerous, Sky News previously reported.

At the time of Kate's hospitalisation, Kensington Palace signalled that it would be unlikely the princess would return to public duties before Easter.