In statement released today, Buckingham Palace said: "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.
King Charles has been diagnosed with an unknown form of cancer that was spotted during his prostate operation.
In a statement, Buckingham Palace said a "separate issue of concern was noted" after Charles spent three nights in hospital last month.
Tests carried out have revealed "a form of cancer", the palace added.
The King, 75, is understood to have notified his siblings and children personally, with Prince Harry now flying to the UK to support his dad.
Reacting to the announcement, Prime Minister Rishi Sunak said: "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."
Opposition leader Sir Keir Starmer added: "On behalf of the Labour Party, I wish His Majesty all the very best for his recovery.
"We look forward to seeing him back to swift full health."