King Charles had a brief trip to the hospital this afternoon after he experienced "temporary side effects" from his ongoing cancer treatment, Buckingham Palace said in a statement. He required a "short observation in hospital" which postponed his afternoon engagements.

The monarch, 76, who was diagnosed with an undisclosed form of cancer at the start of last year, has now returned home to Clarence Home and is understood to be in "good form". A royal source said he continues to work on State papers and make calls from his study.

But today's events have caused the monarch to cancel his planned away day to Birmingham tomorrow. Charles had been due to undertake a busy programme of engagements in the city but will reschedule on medical advice.

A spokesperson said: "His Majesty was due to receive Credentials from the Ambassadors of three different nations this afternoon. 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 rescheduled in due course and offers his deepest apologies to all those who had worked so hard to make the planned visit possible."

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."

The King visited the London Clinic, the hospital where he was admitted for prostate surgery last January and where the Princess of Wales was treated for major abdominal surgery.

His diary is expected to continue as normal next week, though decisions may be taken at the last minute to protect and prioritise the King's continued recovery.

The King and Queen are due to undertake a state visit to Italy from April 7 and the visit is expected to go ahead as planned.