King Charles III, 75, has cancer, Buckingham Palace announced on Monday.
"During The King’s recent hospital procedure for benign prostate enlargement, a separate issue of concern was noted," Buckingham Palace said in a statement. "Subsequent diagnostic tests have identified a form of cancer."
On Monday, Charles started "a schedule of regular treatments, during which time he has been advised by doctors to postpone public-facing duties," the palace said, and during this time he'll "continue to undertake State business and official paperwork as usual."
The palace has not specified the type of cancer, the stage of cancer or the type of treatment.
"The King is grateful to his medical team for their swift intervention, which was made possible thanks to his recent hospital procedure," the palace continued. "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 personally told his children and his siblings, according to a royal source.
Prince Harry spoke with his father and is planning to visit him in the United Kingdom in the coming days, ABC News has confirmed.
Queen Camilla will continue with her public duties, according to a palace spokesperson.
One week ago, on Jan. 29, Charles was discharged from The London Clinic after undergoing a procedure to treat an enlarged prostate, according to Buckingham Palace. Charles was expected to resume public engagements after a "short period of recuperation," the palace said at the time.
The palace first shared the news of Charles' medical condition on Jan. 17, announcing that he would be hospitalized for a "corrective procedure." At the time, the king's condition was said to be benign, meaning non-cancerous.
U.K. Prime Minister Rishi Sunak on social media wished Charles a "full and speedy recovery."
President Joe Biden told reporters on Monday he "just tried to call" Charles.
"I’m concerned about him,” Biden said. "I just heard his diagnosis, but I’ll be talking to him, God willing."
Shortly after, Biden posted on social media, "Navigating a cancer diagnosis, treatment, and survivorship takes hope and absolute courage. Jill and I join the people of the United Kingdom in praying that His Majesty experiences a swift and full recovery."
Navigating a cancer diagnosis, treatment, and survivorship takes hope and absolute courage.
— President Biden (@POTUS) February 5, 2024
Jill and I join the people of the United Kingdom in praying that His Majesty experiences a swift and full recovery.
Canadian Prime Minister Justin Trudeau wrote on social media he is "thinking of" Charles "and hoping for a fast and full recovery."
French President Emmanuel Macron shared his well-wishes for the King on X Monday.
"Wishing His Majesty King Charles III a speedy recovery. Our thoughts are with the British people. Amitiés," he wrote.
Meanwhile, the king's daughter-in-law, Kate, the Princess of Wales, is recovering from a Jan. 16 "planned abdominal surgery", according to Kensington Palace.
The surgery was "successful," according to the palace. Kate, 42, was discharged from the hospital on Jan. 29 after a nearly two-week stay. Kate, who is married to Prince William -- Charles' son and heir to the throne -- and is a mom of their three young kids, is now recovering at home.
Charles was 73 years old when he became king in September 2022, following the death of his mother Queen Elizabeth II, the longest-reigning monarch in British history.
ABC News' Katie Kindelan and Zoe Magee contributed to this report.
