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.