The King has been diagnosed with a form of cancer, Buckingham Palace has announced.
The Palace confirmed the disease, which is not prostate cancer, was discovered while the King was being treated in hospital for an enlarged prostate.
The King has started a schedule of regular treatments, but has been advised to postpone his public-facing duties.
The Palace declined to confirm the type of cancer.
He will carry on working behind the scenes on state business and official papers, and returned from Sandringham to London on Monday to commence treatment as an out-patient.
Buckingham Palace said in a statement: “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.”
Charles, 75, was discharged from the London Clinic a week ago after undergoing treatment on an enlarged prostate.
He acceded to the throne just 17 months ago, and was last seen on Sunday when he attended church in Sandringham.
The King has visited York twice in his reign already. He and the Queen came to the city in November 2022 to unveil a statue of his mother, the late Queen Elizabeth II, on York Minster.
The King then returned on 6 April last year, to distribute the Maundy Money at the Minster.
And last June, he visited Pickering – taken there by Flying Scotsman.
The Prime Minister has wished the King a “full and speedy recovery” and said he will no doubt “be back to full strength in no time”.