King Charles was hospitalized for “a short period of observation” on Thursday after experiencing “temporary side effects,” related to a scheduled cancer treatment, Buckingham Palace said in a statement.

His engagements for Thursday afternoon and Friday were cancelled.

“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,” the palace said.

“His Majesty would like to send his apologies to all those who may be inconvenienced or disappointed as a result.”

Charles, 76, has been undergoing treatment for an undisclosed form of cancer for more than a year. The King has continued fulfilling his state duties, even though he took some time off public duties.

His illness came as his daughter-in-law, Catherine, the Princess of Wales, was also diagnosed with cancer. She took more than six months off before returning to public duties in late September.