The King cancelled an away day to Birmingham yesterday 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 "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."
The palace said the King had returned to Clarence House and as a precautionary measure and acting on medical advice had rescheduled his diary programme.
"His Majesty would like to send his apologies to all those who may be inconvenienced or disappointed as a result."
The King was diagnosed with an undisclosed form of cancer in February 2024 and returned to public-facing duties in April despite still undergoing weekly treatment. — PA/dpa
