King Charles will be formally proclaimed the new monarch in an Ascension Council ceremony, which will be televised for the first time.
While Charles became King automatically upon the death of his mother Queen Elizabeth, his role will be confirmed at the meeting attended by Privy Councillors on Saturday morning.
King Charles will not attend the ceremony, which will be held at the State Apartments of St James’ Palace, however will join after he has been proclaimed monarch at 10am to hold his first Privy Council meeting.
It comes after the new King addressed the nation for the first time on Friday, paying tribute to his “darling Mama” who died on Thursday afternoon at Balmoral.
In the address, he pledged his whole life to serve as the new sovereign as his mother did, saying: “That promise of lifelong service I renew to you all today”.
King Charles also confirmed in the speech that he had made his son and heir apparent Prince William the new Prince of Wales, with his wife Catherine the first to use the title Princess of Wales since the death of her late mother-in-law Diana.
The new King also paid tribute to his younger son Prince Harry and his wife Meghan in the address, expressing “love for Harry and Meghan as they continue to build their lives overseas”.
King Charles also thanked his “darling wife” Camilla, in the speech, calling her “my Queen Consort”, saying: “I know she will bring to the demands of her new role the steadfast devotion to duty on which I have come to rely so much.”
The Proclamation ceremony will feature a platform party, made up of Queen Camilla, the Prince of Wales, 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, who will sign the Proclamation.
During the event the Lord President will announce the death of the sovereign and call upon the Clerk of the Council to read aloud the text of the Accession Proclamation.
It will include Charles’s chosen title as King, already known to be King Charles III.
At 11am a Principal Proclamation will be read in public for the first time by the Garter King of Arms in the open air from the balcony overlooking Friary Court at St James’s.
It will be followed by a flurry of Proclamations around the country, with the second one at City of London at the Royal Exchange at midday on Saturday, and further Proclamations in Scotland, Northern Ireland and Wales at midday on Sunday.
In recognition of the new Sovereign, union flags will be flown at full-mast from the time of the Principal Proclamation at St James’s Palace until one hour after the Proclamations in Scotland, Northern Ireland and Wales, after which flags will return to half-mast in mourning for the death of the late Queen.
The ceremony is being staged a day later for King Charles III than normal practice because the announcement of the Queen’s death did not come until early evening on Thursday, meaning there was not enough time to set the plans in motion for Friday morning.
The Palace said: “His Majesty The King will be proclaimed at the Accession Council at 10.00am (on Saturday) in the State Apartments of St James’s Palace, London.
“The Accession Council, attended by Privy Councillors, is divided into two parts.
“In Part I, the Privy Council, without The King present, will proclaim the Sovereign, and formally approve various consequential orders, including the arrangements for the Proclamation.
“Part II, is the holding by The King of His Majesty’s first Privy Council.
“The King will make his Declaration and read and sign an oath to uphold the security of the Church in Scotland and approve Orders in Council which facilitate continuity of government.”