The King will continue with state business and official paperwork while he is being treated for cancer, Buckingham Palace has said.
But as he steps back from public engagements, there are questions about how his duties might change.
The King is a constitutional monarch who must remain politically neutral.
The head of state has “an important formal and ceremonial relationship” with Parliament, the monarchy’s official website says.
He holds a regular audience with the Prime Minister, usually on a Wednesday at Buckingham Palace.
This private weekly meeting with Rishi Sunak is expected to continue, and may happen remotely if medics advise the King to reduce in-person contact.
He will also continue to receive and review important documents in the red box used by the monarch and Government ministers.
Charles is also expected to remain available for meetings of the Privy Council, which usually meets once a month.
This is the oldest form of legislative assembly still functioning in the UK, responsible for a number of executive responsibilities.
At each meeting the Council obtains the King’s formal approval of orders which have already been discussed and approved by ministers.
He also approves proclamations through Council, formal notices covering issues such as the summoning of a new Parliament, coinage and the dates of certain bank holidays.
The King’s role in general elections will be a focus, as Mr Sunak has hinted he will send the country to the polls to elect a new Westminster government in the second half of this year.
The monarch dissolves Parliament before a general election.
The day after the vote, it is his duty to invite the leader of the party that won the most seats in the House of Commons to become Prime Minister and to form a government – one of the few remaining personal prerogatives of the sovereign.
It was one of the Queen’s last duties, just two days before she died, when she appointed Liz Truss prime minister, at Balmoral Castle.
It was the first time the Queen, who had mobility issues, carried out the key duty at her Scottish retreat rather than at Buckingham Palace.
Another one of the King’s official parliamentary roles is assenting to Bills passed by Parliament on the advice of ministers.
Legislation must receive his sign-off to become law. Royal Assent has not been refused since 1707.
It is also a long-established convention that the monarch is asked for consent to debate Bills which would affect the prerogative or interests of the Crown.
The King also opens Parliament in the annual State Opening ceremony and sets out the Government’s agenda in the King’s Speech.
Charles’ first State Opening of Parliament as King was in November, although he delivered the last Queen’s Speech of Elizabeth II’s reign in his mother’s place in 2022 after she dropped out due to health issues.
Traditionally the Lord Chancellor would read the speech on the sovereign’s behalf if he is unavailable.
He can summon new Parliaments on the Government’s advice, and open and close, or prorogue, each session of Parliament.
The King also hosts heads of state visiting the UK.