Meanwhile, the Duke of Sussex was pictured at London's Heathrow Airport on Wednesday afternoon following his brief reunion with his father on Tuesday.
Prince Harry, 39, flew to the UK less than 24 hours after the announcement about the King’s health was made to the nation by the palace. The Duchess of Sussex remained at the couple's home in Montecito, California with their children, Prince Archie, four, and Princess Lilibet, two.
Prince William, 41, hosted an investiture ceremony at Windsor Castle on Wednesday morning before attending a gala fundraising dinner for London's Air Ambulance Charity in his role as the organisation’s patron.
The father-of-three had temporarily stepped back from royal duties when Kate was admitted to hospital on 16 January, in order to juggle caring for his wife and their children.
The Princess spent 13 days at the London Clinic – the same private hospital where Charles received treatment for an enlarged prostate.
On Tuesday, Buckingham Palace confirmed the King, who only acceded to the throne 17 months ago, does not have prostate cancer, saying only that he has a “form of cancer”.
He was diagnosed after a “separate issue of concern was noted” and investigated while he was being treated for his benign enlarged prostate.
The 75-year-old King was seen in public for the first time since his cancer diagnosis on Tuesday as he left Clarence House the day after starting his treatment. Charles and Camilla later travelled to Sandringham by helicopter.
Prince William attends gala
The Prince of Wales has arrived at the London's Air Ambulance Charity Gala. William looked incredibly suave as he arrived in his tuxedo. As the Prince of Wales arrived, he thanked those gathered for their "well-wishes" following his father's cancer diagnosis. "We really appreciate everyone’s kind messages, thank you," he said.
Also in attendance was Tom Cruise, and William was seen smiling and laughing with the actor as they shared a moment together.
In his speech, William shared: "I'd like to take this opportunity to say thank you, also, for the kind messages of support for Catherine and for my father, especially in recent days. It means a great deal to us all."
Prince William pictured for the first time
The Prince carried out the investiture at Windsor Castle on Wednesday morning – which is one of his own regular duties – where former England footballer Ellen White was among those receiving honours.
William, who is president of the Football Association, made the record-scoring Lioness an MBE for services to football.
Ellen White's tribute to King Charles
Asked about her conversation with the prince, Ellen told the PA news agency: "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."
She added: "It was a really nice conversation. I’ve met him a couple of times, he definitely puts you at ease."
Other recipients due at the ceremony include David Shreeve – who co-founded an environmental action group with TV botanist David Bellamy – and Francis Dickinson – who solved an 80-year-old maritime mystery by locating the wreck of a submarine which was lost at sea during the Second World War.
Tonight's gala dinner for London's Air Ambulance Charity will raise funds for its Up Against Time appeal, which is seeking £15 million to replace the service’s helicopter fleet by the autumn.
Prince Harry did not reunite with Prince William during brief UK visit
Harry flew thousands of miles from his Californian home to be at the King’s side, meeting for around 45 minutes at Clarence House on Tuesday afternoon.
But as William resumed his public duties on Wednesday, Harry was then pictured at Heathrow Airport ready to return to the US.
There were no reunion between the royal brothers on this occasion. William and Harry were last pictured together at the King's coronation last May
King Charles to continue weekly audiences with Prime Minister
Prime Minister Rishi Sunak will have his weekly audience with the King over the phone following Charles’ cancer diagnosis, No 10 said.
An official spokesman said: “We don’t in general and we are not going to get into the habit of commenting on the PM’s conversations with the King.
“But we have agreed with the Palace in this specific instance to confirm that they will be speaking on the phone later.”
Prime Minister Rishi Sunak told the Commons earlier today: “I know the thoughts of the House and the country are with the King and his family. We wish His Majesty a speedy recovery and look forward to him resuming his public-facing duties in due course.”
Prince William's last public engagement
The Prince of Wales made his last public appearance on 11 January, travelling 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 receiving the CBE, Burrow said: "It is an honour to accept on behalf of the whole MND community. Days like today allow us to keep the spotlight on the disease and continue to raise awareness and, hopefully, funds to fight the disease and support those who are living with MND and their families.
"It was wonderful that His Royal Highness was able to come to Headingley to see us and very much appreciated. It was lovely to talk to him and for me to meet my family."