King Charles III has been diagnosed with a form of cancer and has begun treatment, Buckingham Palace said on Monday.
"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," the palace said in a statement, adding that he had begun treatment.
The king "remains wholly positive" and "looks forward to returning to full public duty as soon as possible", it said.
Earlier, Prince William returned to royal duty this week after taking time off following his wife's abdominal surgery, Kensington Palace said on Monday.
William, 41, the eldest son of King Charles III and heir to the throne, postponed public engagements to care for his wife Catherine, Princess of Wales and their three children after she was admitted to hospital on January 16.
William will conduct an investiture at Windsor Castle on Wednesday and attend the London Air Ambulance annual fundraising gala in central London on Wednesday evening, the palace said.
Britain's royal family had a double health scare in January when it was announced that Charles would be treated for an enlarged prostate and the Princess of Wales was recuperating from abdominal surgery.
The King left hospital on January 29, three days after his surgery and just hours after Kensington Palace announced that the 42-year-old princess had also left the clinic after undergoing surgery.
Their absence created a staffing shortage within the slimmed-down monarchy with Queen Camilla, 76, the most high-profile public face of the monarchy during her husband's absence.
Charles' recuperation could last until the end of February, according to media reports, while Kate is expected to be out of action until at least March 31.
The King won plaudits for being open about his condition. No reasons were given for Kate's admission to hospital, although officials said it was not linked to cancer.