Prince Harry is set to rush back to be with Meghan Markle as the couple have a major joint appearance scheduled for next week.
The Duke of Sussex flew from Los Angeles to London on Tuesday to see his father.
The King told Harry about his diagnosis during a private conversation on Monday before his cancer was announced publicly.
Harry immediately decided to travel to England after learning of his father's cancer and arrived in London on Tuesday afternoon.
The Duke of Sussex received a police escort from Heathrow Airport to Clarence House following his arrival.
The King spent less than an hour with his son before travelling to Sandringham with his wife Queen Camilla.
Prince Harry is not expected to see his brother Prince William during his stay in the UK.
The Duke of Sussex is scheduled to join Meghan Markle at an event to commemorate a year until the Invictus Games in Vancouver and Whistler.
Harry founded the Invictus Games as a working member of the Royal Family and has continued to champion the sporting event despite stepping down from the monarchy in 2020.
The royal and Meghan Markle produced the series Heart of Invictus as part of their Netflix deal.
The five-episode programme followed the 2022 Invictus Games which were hosted in The Hague.
Prince Harry featured throughout the series but it failed to reach the same heights as the Sussexes' debut Netflix documentary, the self-titled, Harry & Meghan.
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The Duchess of Sussex did not travel to the UK with her husband to see the King.
Meghan chose to remain in the US with their her children Prince Archie and Princess Lilibet.
She did not visit the UK in 2023 and was last in the UK for Queen Elizabeth II's funeral in September 2022.