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.
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5:21
‘Serious statement from Palace’
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.”
The head of state underwent a procedure for an enlarged prostate, spending three nights in hospital in late January.
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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.”
A new level of openness about royal medical matters
The King looked well as he walked to church at Sandringham on Sunday which makes this announcement even more unexpected.
The palace is keen to stress he remains wholly positive about his treatment but inevitably aware that his diagnosis will cause concern.
It feels like a complete change from what we would expect from the Royal Family with a new level of openness about their medical matters.
With the Queen and Prince Philip, the amount of detail was always much more scarce.
So why the change of tack?
There is an acknowledgment that as our head of state we should know about something this serious.
But it’s clear the palace and the monarch wanted to make sure they were in control, as much possible, when it came to revealing his cancer diagnosis to the world.
A kind of deal has been done. We’ve let you in on what’s going on, now it’s time to give the King time away from the cameras for this period of treatment and recovery.
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.
Image: Prince Harry is due to see his father in the coming days. Pic: AP
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.
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1:38
What next for the King?
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.