The King promised to serve the nation with the same "unswerving devotion" as the late Queen had
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.
The King's accession will be proclaimed in the City of London's Royal Exchange, as his mother's was more than 70 years ago
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.
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".
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".
-- Courtesy of BBC News