The type of cancer has not been revealed – it is not prostate cancer, but was discovered during his recent treatment for an enlarged prostate.
The King began “regular treatments” on Monday and will postpone public duties during it, the Palace said.
The Monarch, 75, “remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible,” it added.
No further details are being shared on the stage of cancer or a prognosis.
Charles informed both his sons personally about his diagnosis and the Prince of Wales was said to be in regular contact with his father.
The Duke of Sussex, Prince Harry, who lives in the United States, spoke to his father and will be travelling to the UK to see him in the coming days.
The King returned to London from Sandringham in Norfolk on Monday morning and the palace says he has started treatment as an outpatient.
Although he will pause his public events, the King will continue with his constitutional role as head of state, including paperwork and private meetings.
It is understood his weekly audiences with Prime Minister Rishi Sunak will continue and will be in person, unless doctors advise that he limits such contact.
There is a constitutional mechanism for when the head of state is unable to carry out official duties – in that circumstance “counsellors of state” can be appointed to stand in for the monarch.
At present that includes Queen Camilla, Prince William, the Princess Royal, and Prince Edward. Prince Harry and the Duke of York are no longer called upon as they are non-working royals.
Prince William had also temporarily withdrawn from public engagements while he helped his wife Catherine, the Princess of Wales, as she recovered from abdominal surgery she had last month.
But it was announced earlier on Monday that he would return to public duties later this week.
BBC
