King Charles will formally be declared Monarch on Saturday with Prince William by his side in the first-ever televised Accession ceremony.
New Prime Minister Liz Truss, her cabinet members and other dignitaries are among those gathering for the historic event.
The royal automatically became King following the tragic death of his mother on Thursday but his role will be confirmed this morning.
A ceremony will be carried out by the Accession Council at St James’ Palace, which will not be attended by King Charles III.
He will then join at 10am to hold his first Privy Council meeting where he will make a personal declaration about the death of the Queen.
Prince William, Queen Consort Camilla, the Archbishop of Canterbury, and the Prime Minister will be among those who sign the Proclamation.
Flags lowered in mourning for the Queen will fly at full-mast again after the historic meeting.
At 11am, the first public proclamation will be heralded from the Friar Court balcony of the Palace as gun salutes ring out at Hyde Park and the Tower of London.
Although not part of the formal proclamation, the words “the Queen is dead, long live the King” are often added afterwards.
The formalities come after King Charles last night pledged to follow his “darling mama’s” life of service in his first address to the nation.
He also spoke of his “profound sorrow” at her death and praised the “affection, admiration and respect” the Queen inspired that “became the hallmark of her reign”.
The King spoke of her “unswerving dedication” as he took a “solemn pledge” to uphold the same “constitutional principles” for “the remaining time god grants me”.
And he poignantly touched on the Queen’s “qualities with warmth, humour and an unerring ability always to see the best in people.”
The King ended the speech by saying: “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.
“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’.”
Charles stepped foot on English soil on Friday for the first time as King after flying back from Balmoral.
There were cheers of “God save the King” as he arrived at Buckingham Palace.
He shook hands with well-wishers as he begun his first day as the nation’s monarch after a 70-year wait for the throne.
One over-eager supporter even gave the King a kiss on the cheek in a breach of protocol.