It comes after Queen Elizabeth II died peacefully at Balmoral on Thursday afternoon (8 September) aged 96.
The Queen’s son and successor will now turn his attention to matters of state as he begins his first full day as the nation’s new monarch.
Here is everything you need to know about the accession process and how King Charles III will be coronated.
What is the Accession Council?
The Accession Council assembles only upon the death of a monarch and its role is to formally confirm an accession of a new sovereign succeeding to the throne.
The last time the Accession Council met was in February 1952 to officially mark Elizabeth II taking the throne.
The council will next meet in St James’ Palace in London on Saturday (10 September) to proclaim King Charles the new sovereign of the United Kingdom.
The Accession Council is usually convened within 24 hours of the death of a Sovereign, traditionally at 10am, and customarily meets at St James’s Palace to make formal Proclamation of the death of the Monarch and the accession of the successor to the throne.
Historically, the entire Privy Council is summoned to the Accession Council to oversee the formal proclamation of a new monarch. However, with the number of privy counsellors – who are lifetime members and mostly past and present politicians – now standing at more than 700, restrictions have been put in place.
Just 200 privy counsellors will be summoned and those who are cut will be asked to enter an annual ballot for a few remaining seats.
The council must take place before parliament meets, and parliament should meet as soon as practicable after the death of a sovereign.
What is the Privy Council?
The Privy Council was originally the executive arm of the English Government from the 13th century. Although its power declined in the late 1600s due to rise of the Cabinet.
It remains an advisory body to the monarch, and its members are known as Privy Counsellors.
All Cabinet members become Privy Counsellors, and you will hear them in the House of Commons be referred to as the “Right Honourable”.
Counsellors are appointed for life, so all former Prime Ministers are members, as well as the Archbishops of Canterbury and York and the First Ministers of Scotland, Wales, and Northern Ireland.
Overall there are more than 700 Privy Counsellors.
The main function is to advise the monarch on giving formal effect to proclamations - such as the proclamation of the new King, and orders in council, legal instruments which are made under prerogative or statutory powers.
How does the Accession Council work?
The Accession Council is divided into two parts and is presided over by the Lord President of the Council, who has ministerial responsibility for the Privy Council Office.
Penny Mordaunt was appointed Lord President of the Council, and Leader of the House of Commons, on 6 September in Prime Minister Liz Truss’s new cabinet, in place of Mark Spencer, with the Queen officially approving the appointment.
Ms Mordaunt is yet to be “declared” Lord President at a Privy Council meeting because the event was postponed on Wednesday (7 September) when the Queen was urged to rest.
Part 1 - The Proclamation
The chosen Privy counsellors – without the King – will gather at St James’s Palace to proclaim the new sovereign, and will be joined by Great Officers of State, the Lord Mayor and City Civic party, Realm High Commissioners and some senior civil servants.
If any of the counsellors summoned are not able to attend at short notice, the Council can still take place.
Camilla – the new Queen – and the Duke of Cornwall and Cambridge are already privy counsellors so will be present.
The Lord President announces the death of the sovereign when the meeting begins and will call upon the Clerk of the Council to read aloud the text of the Accession Proclamation. This will include Charles’s chosen title as King, which is already known to be King Charles III.
The platform party - made up of Camilla and William, the Archbishop of Canterbury, the Lord Chancellor, the Archbishop of York, the Prime Minister, the Lord Privy Seal, the Lord Great Chamberlain, the Earl Marshal and the Lord President – sign the Proclamation.
The Lord President then calls for silence and reads the remaining items of business, which deal with the dissemination of the Proclamation and various orders giving directions for firing guns at Hyde Park and the Tower of London.
Part II – The King’s First Privy Council
Charles then enters and holds his first Council, which is only attended by Privy Counsellors only.
During this he will first make a personal declaration about the death of the Queen and will also take the oath to preserve the Church of Scotland, because there is a division of powers between Church and State in Scotland.
He will read it out loud and sign two identical Instruments recording the taking of the oath, with his signature witnessed by Camilla and William, and others including the Secretary of State for Scotland and the Scottish First Minister.
Another oath, the Accession Declaration, to maintain the protestant succession, is usually made several months later at the State Opening of Parliament.
Other business will be dealt with, including the use of the Seals, to “facilitate the continuity of government”, and Privy counsellors will sign the Proclamation as they leave St James’s Palace.
The official record of proceedings will be published in a special supplement to the London Gazette.
What happens next?
After the Accession Council, the first public proclamation of the new sovereign is read in the open air from the Friary Court balcony by the Garter King of Arms at St James’s Palace in the presence of the Earl Marshal and two of the sovereign’s Serjeants at Arms.
Trumpeters usually play a fanfare from the balcony and gun salutes are fired in Hyde Park and at the Tower of London at the same time.
The Proclamation will then be read at the Royal Exchange in the City of London, as well as publicly in other cities across the UK, including Edinburgh, Cardiff and Belfast. It is also usually read at Windsor and in York, where the mayor traditionally drinks to the new sovereign’s health from a golden goblet.