Televised for the first time in its history, the Accession Council meeting at 10am will show the King being formally proclaimed monarch with Charles making his declaration and oath.
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 an hour later at 11am.
It will be followed by a flurry of proclamations around the country, with the second one in the City of London at the Royal Exchange at midday on Saturday, and further proclamations in Scotland, Northern Ireland and Wales at midday on Sunday.
The momentous occasion follows the death of Queen Elizabeth II, 96, at Balmoral Castle on Thursday. The passing of Britain’s longest-reigning monarch has drawn an outpouring of tributes from across the UK and around the world.
On Friday evening, King Charles paid a heatfelt tribute to his late mother Queen Elizabeth, vowing to serve with “loyalty, respect and love” as she had done for more than seven decades.
“I speak to you today with feelings of profound sorrow,” said Charles, who became King on his mother’s death.
“Throughout her life, Her Majesty The Queen – my beloved Mother – was an inspiration and example to me and to all my family, and we owe her the most heartfelt debt any family can owe to their mother; for her love, affection, guidance, understanding and example.”
Charles and other members of the royal family had rushed to be at the Queen’s side when news of her ailing health emerged.
He returned to Buckingham Palace in London on Friday where he was greeted with cheers, applause and a crowd singing “God Save The King” as he made his first public appearance since ascending the throne.
In his televised message, Charles reflected on his mother’s promise she made in 1947, aged 21, to devote her life to the service of her peoples.
“That was more than a promise: it was a profound personal commitment which defined her whole life. She made sacrifices for duty,” he said.
“As the Queen herself did with such unswerving devotion, I too now solemnly pledge myself, throughout the remaining time God grants me, to uphold the Constitutional principles at the heart of our nation.
“And wherever you may live in the United Kingdom, or in the Realms and territories across the world, and whatever may be your background or beliefs, I shall endeavour to serve you with loyalty, respect and love, as I have throughout my life.”
Charles, 73, also said he had made his eldest son William the new Prince of Wales, the title that had been his for decades and one traditionally held by the heir to the throne. William’s wife Kate becomes Princess of Wales, a role last held by the late Princess Diana.
“With Catherine beside him, our new Prince and Princess of Wales will, I know, continue to inspire and lead our national conversations, helping to bring the marginal to the centre ground where vital help can be given,” he said.
Britain has declared a period of mourning until the state funeral for Elizabeth, once described by her grandson Harry as “the nation’s grandmother”, which will be held in a little over a week’s time.
Thousands gathered at royal palaces to pay their respects, with some shedding tears as they laid flowers.
“She was amazing. She was like everyone’s granny,” said Kay McClement, 55, who came with a friend to leave flowers at Balmoral Castle.
Railway worker Liam Fitzjohn, 27, said he brought his daughter for a moment of history. “She’s all we ever knew,” he said.
The House of Commons will also convene on Saturday to approve a message of condolence to the King.
“Since last night’s shocking news, we have witnessed the most heartfelt outpouring of grief at the loss of her late majesty, the queen,” Prime Minister Liz Truss told MPs, who held a minute’s silence at the start of proceedings.
“She was the rock on which modern Britain was built,” said Ms Truss, who Elizabeth appointed just on Tuesday in her last public duty - the 15th premier of her long reign.
The Queen had been suffering from what Buckingham Palace had called “episodic mobility problems” since the end of last year, forcing her to withdraw from most of her public engagements. Her husband of 73 years, Prince Philip, died in April last year.
In his address, King Charles recognised that his life would now change.
“It will no longer be possible for me to give so much of my time and energies to the charities and issues for which I care so deeply. But I know this important work will go on in the trusted hands of others,” he said.
He concluded with a message directly to his late mother.
“To my darling Mama, as you begin your last great journey to join my dear late Papa, I want simply to say this: thank you,” Charles said.
“Thank you for your love and devotion to our family and to the family of nations you have served so diligently all these years. May ‘flights of Angels sing thee to thy rest’.”