King Charles III has been diagnosed with cancer and has begun treatment, Buckingham Palace said on Monday, sparking a flood of support from around the world.

Charles, who became king upon the death of his 96-year-old mother, Queen Elizabeth II, on Sept. 8, 2022, has generally enjoyed 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."

"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's two sons with the late Princess Diana, who now lives in California with his wife Meghan, said he would travel to see the king in the UK over the coming days, a source close to Harry 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 X, formerly Twitter.

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 hospitalization 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 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's 76-year-old second wife, as the most visible face of the royal family.

Royal expert Joe Little of Majesty Magazine said Camilla would be a "massive support" to her husband.

"Camilla has shown for a long time, and we're very aware now, that she is famed as the power behind the throne," he said.

Little added that the royal family took a "business as usual" approach when it came to personal health matters.

"I think they are keeping calm and carrying on, as they would be expected to," he said. (AFP)