The Queen has said the King is "doing extremely well", as the 75-year-old monarch continues his cancer treatment.
The Queen had left the King at Sandringham to fulfil her royal engagement and among the first people she met were medical staff from Wiltshire Air Ambulance.
Ben Abbott, a critical care paramedic, said: "I do hope His Majesty is doing well Ma'am, we're all really sad to hear the news."
In her first public words since the diagnosis was announced on Monday, she replied: "He is doing extremely well under the circumstances.
"He is very touched by all of the letters and messages the public have been sending from everywhere. That's very cheering."
The Duke of Sussex returned to his family in California after he is said to have spent about 45 minutes with his father at Clarence House.
King Charles was diagnosed with a "form of cancer" after he had hospital treatment for an enlarged prostate.
Buckingham Palace has not confirmed what type of cancer he has. But it has said he does not have prostate cancer.
Meanwhile, Prince William returned to royal duties for the first time following his wife's abdominal operation and the King's diagnosis.
Arriving at a gala for the London Air Ambulance Service in central London on Wednesday night, the Prince of Wales waved to people gathered outside and said: "We really appreciate everyone's kind messages, thank you."
Inside, he gave a speech and again thanked everyone for "kind messages".
The King has postponed all public-facing duties, although he is continuing with work on his red boxes of state papers.
Prince William may step in to represent his father at an event, but no engagements are scheduled and arrangements will be made if the need arises.
The Princess of Wales left hospital last Monday - a week before the King's diagnosis was made public - after abdominal surgery.
She returned to Adelaide Cottage in Windsor to be reunited with her three children, but 42-year-old Kate is not expected to return to official duties until after Easter.
The Queen has a home in Lacock in Wiltshire and she chatted with the air ambulance medics about their work.
Mr Abbott said: "I told her we always wave when we fly over Lacock and she said she always waves back."