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Charles III will be formally proclaimed king at a historic ceremony at St James's Palace on Saturday morning.
Flags lowered in mourning for the late Queen will fly full-mast after the Accession Council, which will be televised for the first time.
A wave of further proclamations will take place across the UK until Sunday, when flags will return to half-mast.
It comes after the King pledged to follow his "darling mama's" life of service in an emotional first address.
He told the nation on Friday evening of his "profound sorrow" at the loss of his mother, praising her warmth, humour and "unerring ability always to see the best in people".
The King promised to serve the nation with the same "unswerving devotion" as the late Queen had during her 70-year reign.
Charles became king the moment his mother died, but the Accession Council is held as soon as possible after death of a sovereign to make a formal proclamation of the successor.
At the council, which starts at 10:00 BST, the King will make a personal declaration about the death of the Queen and make an oath to preserve the Church of Scotland - because in Scotland there is a division of powers between church and state.
Among those attending will be Camilla, Charles's wife of 17 years who now has the title of Queen Consort, and the King's son, William, the new Prince of Wales.
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The first public proclamation is due to take place from the Friar Court balcony of St James's Palace in London at 11:00, a moment that is usually accompanied by centuries-old pageantry, with trumpeters playing a fanfare and gun salutes fired in Hyde Park and at the Tower of London.
Although they are not part of the formal proclamation, the words "the Queen is dead, long live the King" are often added afterwards.
The longest continuously serving female MP and Privy Council member Harriet Harman, who will be attending the Accession Council, said the ceremony would be a moment in history.
She told the BBC: "We will all be in St James's Palace and all make the decision that the new monarch should be King Charles.
"Then we do the proclamation, which is telling people of this country that he is their new king. That he is now officially proclaimed."
On Friday, Charles was met by cheers and shouts of "God save the King!" as he met people in the crowds who had gathered at Buckingham Palace.
Later, he gave his televised address, expressing a wish that Prince William and his wife Catherine would - as the new Prince and Princess of Wales - "continue to inspire and lead our national conversation".
- The Accession Council, a formal ceremony to proclaim Charles as king, will be held at 10:00 BST
- From 11:00, flags will fly full mast and royal gun salutes will be fired
- Further public proclamations take place from a balcony at St James's Palace and the City of London
- Senior members of government will swear an oath to King Charles III in the House of Commons
He expressed his "love for Harry and Meghan" and praised the "steadfast devotion to duty" of the Queen Consort.
The King acknowledged that his life had now changed, saying he would not be able to give "so much of my time and energies" to the charities and issues he had supported for decades as heir to the throne.
The 73-year-old monarch said he hoped that despite their sorrow at the Queen's death, people in the UK and the Commonwealth "remember and draw strength from the light of her example".
Former MP Sir Nicholas Soames, who has known the King for 65 years, described the speech as "wonderful".
Speaking to BBC Radio 4 Today programme, he said: "I thought his address was deeply moving and very touching.
"I thought it was completely authentic and, you know, sincerity reigned through it."
What happens at the Accession Council?
- Historically, it is attended by all members of the Privy Council, a body of advisers to the sovereign that dates back to the time of the Norman kings. But with the membership standing at 700, mostly past and present politicians, only 200 are to be summoned.
- They initially gather at St James's Palace without the King. Conservative MP Penny Mordaunt, appointed Lord President of the Privy Council by Prime Minister Liz Truss, will announce the death of the sovereign.
- The clerk of the Council reads aloud the text of the Accession Proclamation, including Charles' chosen title as king - which we know to be Charles III.
- The proclamation is signed by a group including the Queen Consort, the Prince of Wales, the Archbishop of Canterbury, the Lord Chancellor, the Archbishop of York and the prime minister.
- The Lord President calls for silence and reads the remaining items of business, dealing with the public proclamations and orders for the gun salutes at Hyde Park and the Tower of London.
- The King enters for the second part of the council, attended only by privy counsellors and makes a personal declaration about the death of the Queen.
- He takes an oath to preserve the Church of Scotland and signs two documents to record it, with the Queen Consort and the Prince of Wales among those witnessing his signature.
- Privy counsellors will sign the proclamation as they leave.
You can watch The Proclamation of HM the King on BBC One, BBC News Channel and BBC iPlayer from 09:30 BST.
UK Royal Family
King Charles III
Queen Elizabeth II
