King Charles III's cancer was caught early, UK Prime Minister Rishi Sunak said on Tuesday, after treatment began for the monarch who is just 18 months into his reign.
Buckingham Palace has not specified the type of cancer afflicting the 75-year-old monarch, who has stepped back from his public appearances but continues his other official state duties.
The shock announcement sparked an immediate deluge of messages of support, with Charles's estranged son Prince Harry saying he had spoken with his father and would visit him soon.
Charles's diagnosis will prolong a frontline shortage of royals created by Charles's prostate procedure and the almost simultaneous hospitalisation of Catherine, Princess of Wales.
"Thankfully, this has been caught early and now everyone will be wishing that he gets the treatment that he needs and makes a full recovery," Sunak told the BBC.
"I think that's what we're all hoping and praying for and I am, of course, in regular contact with him and will continue to communicate with him as normal," he added.
Charles, who became king upon the death of his mother, Queen Elizabeth II, on September 8, 2022, has generally been reported to be in good health, barring injuries from polo and skiing.
But the palace said that during his recent hospital procedure for benign prostate enlargement, "a separate issue of concern was noted" and that subsequent diagnostic tests had identified "a form of cancer".
"Many families around the country who are listening to this will have been touched by the same thing and they know what it means for everyone," Sunak said.
"So we'll just be willing him on and hopefully we'll get through this as quickly as possible."
Doctors have advised Charles to postpone any engagements although he will continue to "undertake state business and official paperwork as usual", the palace said.
The king "remains wholly positive" and "looks forward to returning to full public duty as soon as possible", it added.
Charles won plaudits last month for being open about his benign prostate condition, with doctors saying many more members of the public had subsequently come forward with symptoms.
He was discharged from a London hospital on January 29 after a three-night stay for corrective surgery.
The palace added that the king had chosen to share his cancer diagnosis "to prevent speculation and in the hope it may assist public understanding for all those around the world who are affected by cancer".
Messages wishing Charles a swift recovery flooded in following the palace statement.
French President Emmanuel Macron wished Charles a "speedy recovery" on X, adding: "Our thoughts are with the British people."
US President Joe Biden said he was "concerned" about King Charles, while Canadian Prime Minister Justin Trudeau said he and the nation were "thinking of his majesty King Charles III as he undergoes treatment for cancer".
Australian Prime Minister Anthony Albanese told reporters: "All Australians will be sending their best wishes to King Charles for a speedy recovery."
Londoners expressed shock and sympathy at the news.
"It's not nice to hear anyone's been diagnosed with cancer. I feel kind of bad for him. He waited all these years to be king," said Sarah Firisen, 55, a software sales worker.
The diagnosis will prolong a frontline shortage of royals created by Charles's prostate procedure and the almost simultaneous hospitalisation of Catherine, Princess of Wales.
Catherine, 42, who is the wife of Charles's son and heir to the throne Prince William, underwent abdominal surgery at the same hospital where Charles was treated.
She left on the same day as Charles, following a stay of around two weeks and may not return to public duties until late March.
Her husband William, 41, also stepped back temporarily from planned engagements to help care for their three children, although he is expected to return to royal duties this week.
At the time of Catherine's operation, Kensington Palace stressed that her condition was not linked to cancer, without elaborating.
The pair's absence left Queen Camilla, Charles's 76-year-old second wife, as the most visible face of the royal family.
