King Charles will postpone his public-facing duties while he is treated for cancer, Buckingham Palace says.
Prince William is expected to cover some duties on his behalf, in addition to his own diary of engagements.
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.
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 the King’s weekly audiences with prime minister Rishi Sunak will continue and will be in person, unless doctors advise that he limits such contact.
The Palace added that Charles “remains wholly positive” about his treatment and “looks forward” to returning to public duty.
The King informed both his sons personally about his diagnosis and the Prince of Wales is 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.