The King hosted the first Buckingham Palace garden party of the season in warm sunshine on Wednesday afternoon.
He stepped out of the palace at 4pm for the traditional national anthem, joined by the Queen and senior members of the Royal family.
Charles, wearing a grey suit and light blue waistcoat, could be seen smiling to the crowds as he entered the garden with the Queen.
He was joined by the Princess Royal, the Duke and Duchess of Edinburgh and the Duke and Duchess of Gloucester.
Just an hour later Prince Harry was to deliver a speech at St Paul’s Cathedral marking the 10th anniversary of the Invictus Games.
But the pair won’t meet during the Duke of Sussex’s flying visit to the capital.
On Thursday the King will meet military staff and their families at a training base for the Army’s Royal Engineers.
Charles, who is Colonel-in-Chief of the Royal Engineers, will visit the 3 Royal School of Military Engineering in Minley, Surrey, before speaking to members of the 8 Engineer Brigade, which is the centre’s deployable unit.
The training base has world-class facilities which include target ranges, swimming pool, state-of-the-art fitness suite and a sailing lake.
It is thought the King has been receiving his cancer treatment as an outpatient midweek.
Charles attended three events last week as he resumed engagements, indicating the positive effect of his treatment.
On Monday, the Lord President of the Privy Council, Penny Mordaunt, said the King is feeling “very good” and “would have been so pleased” to return to public-facing events.
The Privy Council is a body of senior advisers to the King.
Asked how Charles is doing, Ms Mordaunt told GB News: “He’s very good. And I know that he would have been so pleased to get back to public duties. He would have missed it tremendously.
“I think everyone is really pleased to see him back and I know that he wants to do more, so it’s been a good week.”
There was speculation that the Duke of Sussex, who has travelled to the UK to celebrate the 10th anniversary of his Invictus Games, may have seen his father, but a spokesman for Harry said the King’s “full programme” meant the get-together would not happen.
Charles and Harry last met soon after the King announced his cancer diagnosis in February. Harry, who lives with his family in California, travelled from the US to see his father.
A spokesman for the duke said: “In response to the many inquiries and continued speculation on whether or not the duke will meet with his father while in the UK this week, it unfortunately will not be possible due to his majesty’s full programme.
“The duke of course is understanding of his father’s diary of commitments and various other priorities and hopes to see him soon.”
