Britain's King Charles III on Thursday was forced to cancel his appointments for the rest of the day and Friday after suffering "side effects" from his cancer treatment, Buckingham Palace said.
"Following scheduled and ongoing medical treatment for cancer this morning, the king experienced temporary side effects that required a short period of observation in hospital," the statement said.
"His Majesty's afternoon engagements were therefore postponed," the statement added, saying that the 78-year-old UK head of state had since returned to his home in Clarence House.
"As a precautionary measure, acting on medical advice, tomorrow's (Friday's) diary program will also be rescheduled."
The king had experienced temporary and relatively common side effects, the BBC reported, quoting sources, adding that it had been a very minor bump in his recovery.
The king was said to be on good form at home where he was working on state papers and making calls from his study, the PA news agency added.
Charles, who walked most of his life in the shadow of his mother, the nation's beloved Queen Elizabeth II, became king after her death on September 8, 2022.
In a break with the palace's past silence on personal health matters, Charles, however, decided to go public with his cancer diagnosis in February 2024.
But he has never revealed what kind of cancer he is suffering from, with the palace just confirming that his treatment would continue into 2025.
'Apologies'
Buckingham Palace on Thursday said that the monarch "would like to send his apologies to all those who may be inconvenienced or disappointed as a result".
Charles had been due to receive the credentials from the ambassadors of three different countries on Thursday, and had four engagements planned on Friday during a trip to Birmingham.
Charles's coronation in May 2023 as monarch of the United Kingdom and 14 Commonwealth countries was the first in Britain in seven decades.
It also saw his long-time love and his wife of almost 20 years crowned as Queen Camilla -- a once-unthinkable scenario after Charles's first marriage to Princess Diana collapsed in acrimony and scandal.
It is understood that Charles and Camilla's scheduled trip to Italy in early April will go ahead as planned.
No new dates have been confirmed yet after they postponed a planned meeting with Pope Francis at the Vatican following the advice of the pontiff's doctors. The meeting had also been scheduled for the start of the April trip.
The UK's head of state wrote to Francis when the 88-year-old pontiff was struck down with pneumonia in both lungs in February, amid fears that he might not survive.
Francis was discharged on Sunday following almost 40 days in Rome's Gemelli hospital.
'Transparent'
Charles returned to work within two and a half months of his cancer diagnosis and gradually ramped up his duties during the rest of 2024, including making several foreign trips which even took him as far as Australia and Samoa.
The king's daughter-in-law, Catherine, Princess of Wales, who is married to Charles's eldest son Prince William, in January said that she was now in remission after also having been diagnosed with cancer last year.
Buckingham Palace previously said the king wished to be transparent about 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".
That has chimed with his desire for a more modern, open and slimmed down monarchy to keep the ancient institution relevant, particularly to younger Britons, and as republican sentiment mounts in the 14 other countries outside the UK where he is also king.
But a rift with his youngest son Harry and his wife Meghan has blighted the start of Charles's reign.
Harry's score-settling in his autobiography "Spare" and a Netflix series grabbed headlines.
And the prince has only rarely seen his father since he dramatically quit royal duties for a new life in the United States with his wife and young son in 2020.
As well as being head of the Church of England, Charles also heads the 56-nation Commonwealth grouping, which comprises about a quarter of the world's population.
