LONDON (AP) — King Charles III was briefly hospitalized for 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 canceled.

“His majesty has now returned to Clarence House and as a precautionary measure, acting on medical advice, tomorrow’s diary program 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.’’

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, 76, stepped away from public duties 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 1,000-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’s 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, took more than six months off before returning to public duties in late September.

Prince Andrew is still proving to be a problem for his brother, King Charles, despite deliberately stepping back from public life five years ago amid a series of scandals.

The prince, also known as the Duke of York, is back in the spotlight after being linked to an alleged Chinese spy who was banned from the U.K. over national security fears.

According to documents published by a British immigration court on Thursday, the Chinese national secured an “unusual degree of trust” with Andrew, who was “prepared to enter into business activities” with the suspected agent.

Yang Tengbo, the alleged spy known only as “H6” until a High Court judge lifted an anonymity order on Monday, was believed to have carried out “covert and deceptive activity” for the Chinese Communist Party, Britain’s interior ministry said.

Yang denied espionage and said he has done “nothing wrong or unlawful.” The prince said he “ceased all contact” with the businessman after the U.K. government raised concerns.

In any case, the latest round of negative headlines linked to the prince has reportedly led the 64-year-old to “honorably withdraw” from the royal family’s traditional Christmas celebrations to avoid being a distraction.

The saga comes two years after Andrew reached an out-of-court settlement with Virginia Giuffre, who accused the royal in a lawsuit of sexually assaulting and abusing her in 2001 when she was a minor.

Andrew denied the allegations for years. He formally stepped back from royal duties in 2019 following a disastrous interview with the BBC. In the interview, he failed to distance himself from his friendship with the now-deceased sex offender Jeffrey Epstein.

Details about the alleged spying case emerged last week at a special immigration tribunal, which upheld a decision by British authorities to ban Yang from entering the U.K. in 2023.

Judges agreed with MI5, Britain’s domestic intelligence, that Yang “represented a risk to the national security” and dismissed his appeal.

The tribunal heard that in 2021, authorities found documents that showed how close Yang was to Andrew.

Letters from a senior adviser to Andrew “justify the conclusion that [Yang] won a significant degree, one could say an unusual degree, of trust from a senior member of the Royal Family who was prepared to enter into business activities with him,” the 53-page ruling said.

Yang is reportedly a key member of an initiative by Andrew to support entrepreneurs and was invited to the prince’s 60th birthday.

A statement from Prince Andrew’s office said: “The Duke of York followed advice from HMG [His Majesty’s Government] and ceased all contact with the individual after concerns were raised.

“The duke met the individual through official channels, with nothing of a sensitive nature ever discussed. He is unable to comment further on matters relating to national security.”

Yang denied the spying claims. “I have done nothing wrong or unlawful, and the concerns raised by the Home Office [the interior ministry] against me are ill-founded,” he said in a statement. “The widespread description of me as a ‘spy’ is entirely untrue.”