The King is being treated for cancer, Buckingham Palace has announced.
It has not said what type of cancer the 75-year-old has but confirmed that it was not prostate cancer. The King was recently treated for prostate enlargement.
King Charles III was crowned at Westminster Abbey in May 2023 alongside his wife, Queen Camilla.
How will the King's duties change while he is treated for cancer?
"Regrettably, a number of the King's forthcoming public engagements will have to be rearranged or postponed.
His Majesty would like to apologise to all those who may be disappointed or inconvenienced as a consequence."
It said that he was receiving expert care and "looks forward to returning to full public duty as soon as possible."
While the King is recovering, the Queen is expected to continue attending engagements.
"Her Majesty will continue with a full programme of public duties," Buckingham Palace said.
Despite stepping back from public events, the King will continue with paperwork and private meetings as head of state.
The King is the UK head of state, but his powers are largely symbolic and ceremonial, and he remains politically neutral.
He receives daily dispatches from the government in a red leather box, including briefings ahead of important meetings, or documents needing his signature.
The prime minister normally meets the King on a Wednesday at Buckingham Palace.
These meetings are completely private, and no official records are kept of what is said.
The King also has a number of official parliamentary roles:
Appointing a government — the leader of the party that wins a general election is usually called to Buckingham Palace, where they are invited to form a government. The King also formally dissolves Parliament before a general election
State Opening and the King's Speech — the King begins the parliamentary year with the State Opening ceremony, where he sets out the government's plans in a speech delivered from the throne in the House of Lords
Royal Assent — when a piece of legislation is passed through Parliament, it must be formally approved by the King in order to become law. The last time Royal Assent was refused was in 1708
In addition, the monarch leads the annual Remembrance event in November at the Cenotaph in London.
The King also hosts visiting heads of state, and regularly meets foreign ambassadors and high commissioners.
For his first state visit, Charles visited Germany, where he became the first British monarch to address the country's parliament, speaking in English and German.
The King then travelled to France for a three-day state visit in September and to Kenya for a four-day state visit in October, where he acknowledged the "abhorrent and unjustifiable acts of violence committed against Kenyans during their independence struggle."
He also delivered the opening address at the COP28 climate conference in Dubai in December, where he said: "The Earth does not belong to us."
Charles is also head of the Commonwealth, an association of 56 independent countries spanning 2.5 billion people.
He is head of state for 14 of these, known as the Commonwealth realms, as well as the Crown dependencies - the Channel Islands and the Isle of Man.
The Queen supports the King in carrying out his work and undertakes her own public engagements on behalf of the 90 charities she supports.
Where does the Royal Family get its money?
The Royal Family receives an annual payment from the taxpayer, known as the Sovereign Grant, which is used to pay for official expenses, such as the upkeep of properties and staff costs.
The amount is based on a proportion of the profits of the Crown Estate, a property business owned by the monarch but run independently.
It had assets worth £16.5bn in 2022.
The Sovereign Grant was worth £86.3m in 2022-2023, the same as in 2021-2022.
But total spending for the year was £107.5m, a 5% increase on the £102.4m spent the previous year, with more than £20m drawn from financial reserves to cover the shortfall.
The King also receives money from a private estate called the Duchy of Lancaster, which is passed down from monarch to monarch.
It covers more than 18,000 hectares of land, including property in central London.
Worth £654m, it generates about £20m a year in profits.
The Duke of Cornwall (currently William, Prince of Wales) benefits from the Duchy of Cornwall, which mainly owns land in the south-west of England.
Worth £1bn, it generated a net surplus of £24m in 2022-23.
The King and Prince William receive the profits from the duchies personally, and can spend the money as they wish.
Both voluntarily pay income tax on the proceeds.
In addition, some other Royal Family members have private art, jewellery and stamp collections, which they can sell or use to generate income as they wish.
