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The 76-year-old ”required a short period of observation in hospital” on Thursday morning, the palace said.
His afternoon engagements were postponed “as a precautionary measure” and “acting on medical advice”.
King Charles has since returned to Clarence House, his residence in London.
His program for Friday will also be “rescheduled”.
The full statement from the palace, sent on Friday morning in Australia, reads: “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 was due to travel to Birmingham for a full day of engagements on Friday.
His Majesty was taken to the London Clinic, the same place he underwent surgery in January last year for an enlarged prostate.
He was said to be “on good form” at Clarence House, according to The Times, and was working on state papers and making calls from his study.
The 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.”
Buckingham Palace has not revealed the nature of the King’s treatment.
In April last year the monarch returned to public-facing duties after an extended period away on the advice of his medical team.