The King’s appointments for the day were cancelled following his “short period of observation in hospital” on Thursday, Buckingham Palace said.

The King’s health has been closely watched ever since early last year when he announced that he had been diagnosed with an undisclosed form of cancer.

Charles was seen as he left Clarence House in the back of a black Audi.

Tourist Julian Mati said he was relieved to see the King looked well.

“We were horrified when we heard the news yesterday,” Mati said.

“We had come down to the palace today to take pictures but we never imagined we would see the king. To see him smiling and waving, it’s such a relief.”

Charles, 76, stepped away from public engagements for about three months but continued fulfilling state duties, such as reviewing government papers and meeting with the prime minister.

Charles’ cancer diagnosis has heaped pressure on the British monarchy, which is still evolving after the 70-year reign of the late Queen Elizabeth II.

When he succeeded his mother in September 2022, Charles’ task was to demonstrate that the 1000-year-old institution remains relevant in a modern nation whose citizens come from all corners of the globe. But this task takes much time and energy.

Although the duties of a constitutional monarch are largely ceremonial, the royal whirl can be exhausting.

Besides the occasional procession in full royal regalia, there are meetings with political leaders, dedication ceremonies and events honoring the accomplishments of British citizens.

That added up to 161 days of royal engagements during Charles’ first year on the throne.

Charles’ illness came as his daughter-in-law, Kate, the Princess of Wales, was also diagnosed with cancer.

Kate, the wife of Prince William, the heir to the throne, took more than six months off before returning to public duties in late September.