The King's younger son flew in from Los Angeles, where he lives with his wife Meghan and two children. He moved to the US in 2020 after quitting royal duties and falling out with his family.
Prince Harry has visited Clarence House to see his father - a day after the King's cancer diagnosis was revealed.
The Duke of Sussex flew in from Los Angeles Airport in California, where he has lived since 2020, on a British Airways flight and landed at Heathrow Airport in London on Tuesday afternoon.
He was photographed on Monday afternoon being driven into Clarence House, where the King had been recovering after receiving treatment.
Around an hour later, the King and Queen were seen for the first time since his diagnosis was announced.
The royal couple returned to Buckingham Palace and then left by helicopter for Sandringham where they arrived after 4.30pm on Tuesday afternoon.
It is understood that Prince Harry has no plans to see Prince William during his visit to the UK.
The cancer was found after the King underwent treatment for an enlarged prostate - although Buckingham Palace said he does not have prostate cancer.
The King personally called both Harry and the Prince of Wales - as well as his siblings, the Princess Royal, the Duke of York and the Duke of Edinburgh - to share news of his health, the palace said.
Prince William is understood to be in regular contact with his father.
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