As Britain's King Charles III undergoes treatment for a form of cancer, he will withdraw from public-facing duties, with the possibility of sharing responsibilities among other working members of the royal family, while aiming to continue certain key roles.

Buckingham Palace announced on Monday that the king has been diagnosed with cancer and has started receiving treatment.

"No further details are being shared at this stage, but His Majesty is receiving expert care and looks forward to returning to full public duty as soon as possible," a palace spokesperson was quoted by Sky News as saying.

The palace said the treatment began on Monday, but he would continue to undertake state business and official paperwork "as usual".

In the event of the head of state's inability to fulfill official duties, a constitutional mechanism is in place where "counselors of state", including Queen Camilla, Prince William, the Princess Royal and Prince Edward, can be appointed to act as substitutes for the monarch, the BBC reported.

Prince William, a future king and senior royal in the line of succession, will step up to carry out some duties on behalf of his father, while the 75-year-old monarch undergoes treatment, the Press Association reported.

William had temporarily stepped back from his royal responsibilities three weeks ago to attend to the care of his wife Catherine, the Princess of Wales, and their children, after Catherine's abdominal surgery.

The king personally informed both of his sons, as well as his siblings, the Princess Royal, the Duke of York and the Duke of Edinburgh, to give them the news of the cancer diagnosis before it was publicly announced, The Sun newspaper reported.

Prince Harry was due to arrive in the United Kingdom on Tuesday, flying in from California to be with his father.

Since the cancer diagnosis announcement, many world leaders and politicians have wished the king a speedy recovery, along with religious leaders and celebrities who reacted by sending heartfelt messages.

Expressing sadness

In one social media post, Justin Welby, the archbishop of Canterbury, who crowned Charles as king in the coronation ceremony last year, said he is "praying for the king and his family — for God's comfort and strength in the weeks and months to come".

Expressing his shock and sadness, British Prime Minister Rishi Sunak conveyed his gratitude that the king's cancer had been detected at an early stage.

"Obviously, like everyone else, shocked and sad, and just all our thoughts are with him and his family. Thankfully, this has been caught early," Sunak told the BBC on Tuesday.

He added that everyone will be hoping the king "gets the treatment that he needs and makes a full recovery".