The King has been diagnosed with cancer and has started treatment today, Buckingham Palace has announced.
His diagnosis comes following treatment in hospital for an enlarged prostate, although the palace said he does not have prostate cancer.
The palace has not confirmed what type of cancer the 75-year-old monarch has.
The palace said 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.
Heir to the throne Prince William is understood to be in regular contact with his father.
The monarch has begun a schedule of regular treatments and is said to be receiving expert medical care from a specialist team.
But he has been advised by doctors to postpone his public-facing duties.
The King travelled from Sandringham in Norfolk to London this morning to start treatment as an outpatient. He is at home in London this evening, most likely in Clarence House.
He will carry on working behind the scenes on his red boxes - his state business and official papers.
A palace spokesperson said: "His Majesty has been treated for benign prostate enlargement.
"It was during this intervention that the separate issue of concern was noted and subsequently diagnosed as a form of cancer.
"This second condition will now receive appropriate treatment."
It was his first public appearance since being discharged from the London Clinic last Monday.
The head of state underwent a procedure for an enlarged prostate, spending three nights in hospital in late January.
A statement from Buckingham Palace said today: "During the King's recent hospital procedure for benign prostate enlargement, a separate issue of concern was noted. Subsequent diagnostic tests have identified a form of cancer.
"His Majesty has today commenced a schedule of regular treatments, during which time he has been advised by doctors to postpone public-facing duties.
"The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure."
Monarch 'remains wholly positive'
The palace said His Majesty "remains wholly positive about his treatment" and "looks forward to returning to full public duty as soon as possible".
The statement added that the King had chosen to "share his diagnosis to prevent speculation" and "in the hope it may assist public understanding for all those around the world who are affected by cancer".
News of the King's diagnosis comes as his daughter-in-law Kate, Princess of Wales, recovers from abdominal surgery following a hospital stay of around two weeks.
Reaction to King's diagnosis
The King is heading into his cancer treatment as a "very fit 75-year-old" who has always been "very energetic and determined", said Sky's royal commentator Alastair Bruce.
"Everybody knows the impact of the word cancer," he said, adding the monarch's diagnosis gives it "something of a focus".
"I think that having come down to London, we hear, to start this procedure - we know he does so with enormous enthusiasm," he said.
The King has "thrown himself" into his role of head of state and may be "bringing light" to the issue incidentally through his own diagnosis, Bruce continued.
Prime Minister Rishi Sunak reacted to the news of the King's cancer diagnosis by writing on X: "Wishing His Majesty a full and speedy recovery.
"I have no doubt he'll be back to full strength in no time and I know the whole country will be wishing him well."
Labour leader Sir Keir Starmer also issued a get well message to the head of state.