The King is known for being a workaholic, and even the Queen said after his cancer diagnosis that he “won’t slow down and won’t do what he’s told”.

Camilla told author Lee Child at her Reading Room literary festival last year that her husband was “doing fine” but was not heeding her advice.

On Thursday, the King “required a short period of observation in hospital” after experiencing side effects from his cancer treatment.

A busy away day to Birmingham has been cancelled, and his diary could be altered next week, after his medical team advised the King it would be wiser to postpone his engagements as a precautionary measure.

But a hectic state visit to Italy in just 10 days’ time is said to be going ahead.

Following his return from hospital to Clarence House, Charles was said to be working on state papers and making calls from his study on Thursday evening.

When the monarch underwent a procedure on an enlarged prostate before being diagnosed with cancer last year, his wife urged him to slow down and reduce his workload.

But his nephew Peter Phillips said the King was “frustrated” that his recovery after cancer treatment was taking longer than he wanted, and that he was “pushing” his staff to return to his duties. He returned to public-facing events in April.

Camilla has described Charles, now 76 as “not one for chilling”.

The Duke of Sussex once recalled how his father would sometimes fall asleep at his desk and wake up with a piece of paper stuck to his face.

And Lady Sophie Windsor revealed: “You see how he works all day long, has a quick supper and then disappears until about 4am to write letters. He cares about so many things and he comes up with brilliant solutions.”

The King is only less than three years into his reign.

He was the longest serving heir to the throne in British history, and faced the formidable task of succeeding his much-revered mother the late Queen Elizabeth II in 2022.

His cancer diagnosis early in 2024 marked the start of what his eldest son the Prince of Wales described as the most “brutal” year with the Princess of Wales also being diagnosed with cancer.

The King has had a busy run of engagements in the past few weeks including high-profile diplomatic audiences with Ukrainian president Volodymyr Zelensky, a three-day trip to Northern Ireland and packed Palace receptions, amid preparations for the tour to Italy.

An early riser, he usually begins each day with a brisk 11-minute military workout of five basic exercises used by the Canadian air force to keep fit, and after a healthy breakfast, has his first meeting between 8.30am and 9am.

His diary often includes meetings and audiences, or a day out for official visits.

The King skips lunch but does take a break for a walk if possible.

He will also go through his red boxes, containing official and state papers, and have afternoon tea at around 5pm, before resuming work.

After supper in the evening, the King usually works late into the night, and sometimes into the early hours writing letters.

He sees letter-writing as a therapeutic exercise.

He prefers BBC Radio 4 to the television, which Camilla watches in the evening to unwind.

Any rare, spare time is devoted to gardening, or listening to classical music and opera, with his medical team no doubt now urging him to rest as much as possible.