Prince William has referenced the King’s cancer diagnosis for the first time since the announcement by Buckingham Palace on Monday.
Arriving at a gala for the London Air Ambulance Service in central London, the Prince of Wales waved to people gathered outside the venue.
“We really appreciate everyone’s kind messages, thank you,” he said.
Prince William was back at work supporting the King on Wednesday, when he hosted a Windsor Castle investiture ceremony.
Buckingham Palace announced the King had been diagnosed with cancer in a statement on Monday.
Prince Harry had been told, like other senior royals, personally by the King about his diagnosis, and quickly boarded a flight from California, spending around 45 minutes at Clarence House on Tuesday before returning on Wednesday.
The King looked relaxed later that day when, as he returned to Sandringham, he was seen in public for the first time since the diagnosis was announced.
His weekly private audience with the prime minister is planned to be held over the phone, rather than in person.
William is expected to take on more public duties while his father focuses on treatment.
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The gala dinner is raising funds for London’s Air Ambulance Charity’s Up Against Time appeal, which is seeking £15m to replace the service’s helicopter fleet by the autumn.
William is a former air ambulance pilot with the East Anglian Air Ambulance and during the event will meet crew members, former patients and supporters before delivering a short speech.