The King “required a short period of observation in hospital” after experiencing side effects from his cancer treatment, Buckingham Palace has said.
Charles has cancelled an away day to Birmingham on Friday to prioritise his recovery and is now back at Clarence House.
A source described it as a “most minor bump in a road that is very much heading in the right direction”.
Buckingham Palace said in a statement: “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.
“His Majesty’s afternoon engagements were therefore postponed.
“His Majesty has now returned to Clarence House and as a precautionary measure, acting on medical advice, tomorrow’s diary programme will also be rescheduled.
“His Majesty would like to send his apologies to all those who may be inconvenienced or disappointed as a result”.
Charles was diagnosed with an undisclosed form of cancer in February 2024 and returned to public-facing duties in April despite still undergoing weekly treatment.
He has had a busy run of engagements this week including a reception for media, a visit to a soil exhibition, and is due to make a historic state visit to Italy in just 10 days’ time.
The King was due to meet ambassadors in audiences at Buckingham Palace on Thursday afternoon, but these were postponed.
