King Charles Has Cancer. Here’s What to Know.

On Monday night, Buckingham Palace made a sudden announcement that King Charles III had been diagnosed with cancer, less than 18 months after beginning his reign. He is receiving outpatient treatment in London.

The British monarch’s diagnosis prompted an outpouring of sympathy from leaders around the world, with President Biden saying that he was praying for “a swift and full recovery,” and the British prime minister, Rishi Sunak, describing himself as “shocked and sad” at the news. “He’ll just be in our thoughts and our prayers,” Mr. Sunak told BBC radio on Tuesday. “Many families around the country listening to this will have been touched by the same thing.”

Here is what to know about the king’s condition and its implications for Britain’s monarchy.

What do we know about the king’s diagnosis?

Not very much. Buckingham Palace said on Monday night that “a form of cancer” had been diagnosed, but did not state what kind, and asked that reporters not try to contact those involved in Charles’s care.

The palace said that doctors had identified “an issue of concern” while treating Charles, 75, last month for an enlarged prostate. They confirmed cancer — though not prostate cancer — with subsequent tests.

Charles, the statement added, began a schedule of regular treatments on Monday, and “remains wholly positive about his treatment and looks forward to returning to full public duty as soon as possible.”

That the palace has not publicly identified the form of cancer is not a surprise; the British royal family tends to be intensely private about health matters. Three weeks ago, when Charles’s planned prostate treatment was made public, the announcement was seen as a break with tradition.

The palace said that the king, who supported several cancer charities as Prince of Wales, had chosen to share his diagnosis “to prevent speculation and in the hope it may assist public understanding for all those around the world who are affected by cancer.”

What are the king’s normal duties, and who will do them now?

On the advice of his doctors, Charles will temporarily step back from public engagements, which typically include speeches, visits to charities that he supports, community projects and foreign trips.

But the king will continue to carry out his duties as Britain’s largely ceremonial head of state. That includes meeting weekly with Mr. Sunak and tackling a daily “red box” of official paperwork, such as the signing of routine government documents and receiving the credentials of new ambassadors to Britain.

Queen Camilla will carry out a full program of official engagements during her husband’s treatment, the palace said, and Prince William, the heir to the throne, is set to return to public engagements this week. William, 41, had suspended his public duties for a few weeks because his wife, Catherine, was hospitalized in January for abdominal surgery. She is now convalescing at home.

Officials said there were no plans to appoint counselors of state to act in King Charles’s place — a procedure that could signal that the sovereign was unable to fulfill his duties because of illness.

What happens if Charles becomes unable to perform his duties?

If the king is temporarily unable to act as head of state because of illness or traveling abroad, counselors of state are appointed to cover his duties.

Some core constitutional actions still require the king to help formalize them, including dissolving Parliament to make way for new elections; appointing new members to the House of Lords, Parliament’s unelected upper chamber; and appointing a prime minister.

If Charles were to become incapacitated, he could remain king but with a “regent” fully taking on the sovereign’s duties. The regent would be the next in the line of succession: Prince William.

Under the 1937 law that governs the procedure, the king himself would not make the decision. Instead, a group including Queen Camilla and some of Britain’s most senior officers of state, including the head of the English judiciary and the speaker of the House of Commons, would sign a declaration of incapacity, supported by medical evidence.

William would then take a series of formal oaths, including swearing to “be faithful and bear true allegiance to King Charles III, his heirs and successors according to law,” before beginning his new duties.

If the king later recovered, the regency could be ended by a declaration of “cessation of incapacity.”