It is the King’s duty as head of state to appoint a prime minister.
The role is one of the few remaining personal prerogatives of the sovereign – Charles does not act on advice nor need to consult anyone before doing so.
But the overriding requirement is to appoint someone who can command the confidence of the House of Commons – usually the leader of the party with an overall majority of seats in the Commons – to form a government.
Charles could be set for the third prime minister of his reign, if Sir Keir Starmer leads the Labour Party to victory.
His first was Liz Truss – but her stint in charge was the shortest in British history – and the King welcomed Rishi Sunak as his second PM just six weeks after acceding to the throne.
It will be the first time he has appointed a prime minister following a General Election, and the major constitutional duty comes as he continues apace despite his treatment for cancer.
Depending on the election outcome, Charles is set to hold a private audience with the leader of the winning party at Buckingham Palace in London on Friday, the day after the country goes to the polls.
They will be invited by the King to form a government and become prime minister.
The Court Circular, the official record of royal engagements which is published the next day, will record that the politician was requested to form “a new administration” and “kissed hands on appointment” as “Prime Minister and First Lord of the Treasury”.
The kissing of hands is usually just a handshake, with the actual kissing of hands taking place later at a Privy Council meeting.
Any outgoing prime minister will travel to the Palace shortly beforehand to formally tender their resignation in an audience with the King.
In two weeks’ time, the King will be required to carry out the State Opening of Parliament on July 17 – which is also the Queen’s 77th birthday.
He will deliver the King’s Speech, setting out the new Government’s agenda for the coming session, outlining proposed policies and legislation.
The monarch traditionally wears the Imperial State Crown and Robe of State and travels in a carriage procession from Buckingham Palace to the House of Lords for the major ceremonial event, with Camilla at his side.
The King is a constitutional monarch who must remain politically neutral.
He will not be casting his own vote at the ballot box.
Although UK law does not ban royalty from voting, it is considered unconstitutional, so the sovereign does not vote.
The royal family postponed engagements “which may appear to divert attention or distract from the election campaign” after Mr Sunak called a surprise summer election.
One of Queen Elizabeth II’s last duties, two days before she died, was to appoint Ms Truss as prime minister at Balmoral.
It was the first time the Queen, who had mobility issues, had carried out the key duty at her Scottish retreat rather than at Buckingham Palace.
Winston Churchill – with whom she had a close friendship – was her first, and Ms Truss her last.
