King Charles III was briefly hospitalized on Thursday morning after suffering side effects from his cancer treatment, Buckingham Palace said in a statement. The palace said Charles, 76, had since returned to his London residence, Clarence House, and that his public schedule for Friday would be canceled as a precaution.
The king announced in February last year that he had been diagnosed with an undisclosed form of cancer, and he has been undergoing treatment since then. The palace did not specify the nature of the side effects, beyond saying they were temporary and “required a short period of observation in hospital.”
Charles has returned to an active schedule in recent months, playing host to foreign leaders including President Volodymyr Zelensky of Ukraine, and visiting an aircraft carrier, the H.M.S. Prince of Wales.
But the palace has offered no updates on his prognosis or the nature of his treatment. His cancer was discovered during surgery for an enlarged prostate last February; the palace has said that he does not have prostate cancer but has offered no other details about his illness.
That is in contrast to his daughter-in-law, Catherine, Princess of Wales, who also was diagnosed with cancer last year, but announced in January that after several months of chemotherapy, that she was now cancer free.
The palace sought to play down concerns about the king’s condition, saying that his recovery continued on a positive track and that the side effects were not unusual for patients receiving medication. Palace officials said they did not expect Charles to make major changes to his schedule, which has included regular foreign and domestic travel.
The king, for example, plans to go ahead with a state visit to Italy next month. The palace announced earlier this week that he and his wife, Queen Camilla, would not visit Pope Francis at the Vatican because Francis is still recovering from pneumonia and a severe respiratory illness, which kept him hospitalized for more than a month.
“His Majesty was due to receive credentials from the ambassadors of three different nations this afternoon,” the palace said in a statement. “Tomorrow, he was due to undertake four public engagements in Birmingham and is greatly disappointed to be missing them on this occasion.”
“He very much hopes that they can be rescheduled in due course and offers his deepest apologies to all those who had worked so hard to make the planned visit possible,” the statement said.
Concerns about the king’s health have faded in recent months, as he has kept up a vigorous schedule, including a demanding nine-day tour of Australia and Samoa last year. But the dearth of information about his treatment or prognosis has made it difficult to gauge whether he is recovering swiftly or simply pushing himself.
The palace did not rule out further minor adjustments to his schedule, which it said would be made in consultation with his doctors. Officials emphasized that the king relished his public duties and that they contributed to his well-being.
Charles has found himself in the center of a diplomatic swirl in recent weeks. Prime Minister Keir Starmer delivered a formal invitation from the king for President Trump to make a state visit to Britain during his meeting with Mr. Trump in the Oval Office.
Several days later, after Mr. Trump clashed bitterly with Mr. Zelensky, the king played host to the Ukrainian president at his country residence, Sandringham. Mr. Zelensky had traveled to Britain after the United States to attend a meeting of European and other leaders to discuss the war in Ukraine.
Charles also met with two successive prime ministers of Canada, Justin Trudeau and Mark Carney, while they were wrestling with tariffs imposed by Mr. Trump and rejecting his call for Canada to become the 51st American state.
Such meetings are commonplace, given that Charles is also the king of Canada. But they took on added significance in light of the deepening tensions between Canada and the United States.
Even the king’s visit to an aircraft carrier, during which he spoke to the crew and watched fighter jets take off and land on the deck, was laden with symbolism. Britain, along with France, has taken a leading role in trying to assemble a multinational force to help secure a peace settlement in Ukraine.
