Harry flew to London from his home in the US, arriving less than 24 hours after receiving the news of his father's health. His wife, Meghan Markle, and their two children, Archie and Lilibet, did not join him.
The Duke arrived at around lunchtime and was taken to Clarence House to see his father for the first time in almost a year. It was their first proper face-to-face meeting in over a year.
An insider told The Sun that the King is "doing very well" and "entirely his normal self".
"If you didn’t know there was anything wrong with him you wouldn’t know," a source said.
Another adds that the King was "on his usual good form", and "just a little frustrated that his condition has affected not just his own plans but impacted on others".
Royal expert Marlene Koenig told UK newspaper The Express that she didn't believe the very quick meeting would bode well for the family's feud.
"That tells me this was not a successful visit in terms of being able to spend more time with his father," she said. "This is sad."
Harry stayed overnight in a hotel and left the UK on Wednesday afternoon.
Prince Harry has 'no plans' to see Prince William
It comes following reports by the BBC that Harry has no plans to see his older brother Prince William during his stay in the UK.
Another insider revealed to People that there are "no plans" for a reunion between the brothers, with William spending time with his wife Kate Middleton, who is recovering from abdominal surgery.
William will also be taking on some of Charles' responsibilities with the King unable to carry out public-facing duties.
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