Prince Harry is heading back to the US after visiting his father and The Prince of Wales is returning to royal duties following his wife's abdominal operation and the King's cancer diagnosis.
The Duke of Sussex was at Heathrow ready for his return to his family in California after earlier flying into the UK to see the King.
He is said to have spent about 45 minutes with his father, who has started treatment for cancer.
It comes as William, as heir to the throne, hosted his first engagement as the leading member of the Royal Family at official events across the country in the absence of his father.
He has been hosting an investiture ceremony at Windsor Castle, awarding honours like CBEs, OBEs and MBEs to recipients.
Later on Wednesday, he will attend a gala fundraising dinner for London's Air Ambulance Charity.
The King's cancer treatment as an outpatient started earlier this week, and he returned, with the Queen, to his Sandringham home in Norfolk on Tuesday after seeing Harry.
The King has postponed all public-facing duties, although he is continuing with work on his red boxes of state papers.
Meanwhile, Downing Street has said the prime minister will have his weekly audience with the King over the phone.
Kate left hospital last Monday and returned to Adelaide Cottage in Windsor to be reunited with her three children after undergoing planned abdominal surgery on 16 January at the London Clinic.
William last carried out a major royal event more than three weeks ago, when he travelled to Leeds to make former Leeds Rhinos team-mates Rob Burrow and Kevin Sinfield each a CBE for raising funds and awareness for motor neurone disease.
After taking time off work, William had planned to return once Kate's recovery had settled - the 42-year-old future queen is not expected to take on official duties until after Easter.
At the same private hospital that cared for Kate, the King received treatment for an enlarged prostate and was discharged the same day as his daughter-in-law.
When speaking with the Prince of Wales at Windsor Castle, former England footballer Ellen White said she sent her "best wishes" to the King.
William, who is president of the Football Association, made the record-scoring Lioness an MBE for services to football.
Asked about her conversation with the prince, she said:
"I sent my best wishes to the King.
"I just think, you know, it must be a difficult time for the family.
"He was really lovely. It was a really nice conversation. I've met him a couple of times, he definitely puts you at ease."
(c) Sky News 2024: Harry begins journey home to the US - after Prince William hosts first engagement since King's cancer dia
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