King Charles has been diagnosed with a form of cancer, says Buckingham Palace.
It is not prostate cancer, but was discovered during his recent treatment for an enlarged prostate.
The type of cancer has not been revealed, but according to a palace statement the King began "regular treatments" on Monday.
Buckingham Palace says the King "remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible".
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. Throughout this period, His Majesty will continue to undertake State business and official paperwork as usual.
The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure. He remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible.
His Majesty has 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.
Although he will pause his public events, the King, 75, will continue with his constitutional role as head of state.
Prince William has his fingers crossed (or uncrossed) as he is due to return to royal duties on Wednesday for the first time since his wife, Princess Kate, had abdominal surgery last month.
Catherine, 42, spent 13 nights at the London Clinic following the surgery and is not expected to return to royal duties for several weeks.
Queen Camilla has been the most senior royal still in action, carrying out a series of solo events last week, including in London, Bath and Cambridge.
Prince Harry will head to UK to see king after cancer diagnosis.