“Subsequent diagnostic tests have identified a form of cancer,” the palace said in a statement, adding that he had begun treatment, but without elaborating on the type of cancer found or how advanced it was.

The palace said the king “remains wholly positive” and “looks forward to returning to full public duty as soon as possible”.

He had been advised by doctors to postpone public duties, although he would continue to “undertake state business and official paperwork as usual”, the palace added.

The shock announcement sparked messages of support, with Charles’s estranged son Prince Harry saying he had spoken with his father and would visit him soon.

Harry, the younger of Charles’ two sons with the late Princess Diana, who now lives in California with his wife Meghan, plans to travel to see the king in the UK over the coming days, a source close to the prince said.

Charles won plaudits last month for being open about his benign prostate condition, with doctors saying many more members of the public had come forward with symptoms.

He was discharged from a London hospital after a three-night stay for corrective surgery a week ago.

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 shock announcement.

“Wishing his majesty a full and speedy recovery. I have no doubt he’ll be back to full strength in no time and I know the whole country will be wishing him well,” wrote Prime Minister Rishi Sunak on social media platform X.

French President Emmanuel Macron also wished Charles a “speedy recovery” on X, adding: “Our thoughts are with the British people”.

US President Joe Biden said he had “just heard” the news and 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”.

Former US president Donald Trump, the likely Republican nominee in November elections, meanwhile wrote on his Truth Social network that the king was “a wonderful man” and that “we all pray that he has a fast and full recovery!”

Australian Prime Minister Anthony Albanese told reporters that “all Australians will be sending their best wishes to King Charles for a speedy recovery”.

Israel’s Foreign Minister Israel Katz sent “prayers and wishes for the full and fast recovery of the friend of the Jewish people, King Charles”.

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’ son and heir to the throne Prince William, underwent abdominal surgery at the same hospital where Charles was treated.

She left last Monday 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’ 76-year-old second wife, as the most visible face of the royal family.

King Charles departed from royal tradition with his openness about his prostate condition. For centuries Britain’s royal family remained tight-lipped about health matters.

When UK monarchs had real power, news of illness was withheld for fear it might weaken their authority. The habit of secrecy lingered after royals became constitutional figureheads.

The British public wasn’t told that Charles’ grandfather, King George VI, had lung cancer before his death in February 1952 at the age of 56, and some historians have claimed that the king himself wasn’t told he was terminally ill.

In the final years of Elizabeth’s life. the public was told only that the queen was suffering from “mobility issues”. The cause of her death was listed on the death certificate simply as “old age”.

Pat Price, founder of the Catch Up With Cancer campaign, said millions shared the “collective concern” for the king’s health.

“The king’s openness about his battle with cancer is a powerful reminder that one in two of us may face cancer at some point in our lives,” Price said.

Additional reporting by Associated Press