King Charles III has been formally proclaimed monarch during an Accession Council ceremony at St James's Palace, which was televised for the first time.
Following the proclamation senior MPs began the process of pledging their allegiance to the new King.
Commons Speaker Sir Lindsay Hoyle said “time constraints” meant only a small number of MPs could take the oath or affirm on Saturday, but further time will be available at a later date.
Prime Minister Liz Truss was one of the first MPs to pledge her allegiance, she said: “I swear by almighty God that I will be faithful and bear true allegiance to His Majesty King Charles, his heirs and successors, according to law, so help me God.”
As tributes pour in from political figures, senior MPs including the new Prime Minister Liz Truss pledge their allegiance to King Charles III. ITV News Deputy Political Editor Anushka Asthana has the latest
Charles had automatically become King on the death of his mother, but the Accession Council, attended by Privy Councillors, confirmed his role on Saturday morning.
During a poignant and sombre meeting of the Council, the King spoke movingly about his mother and the grief his family is experiencing, but said the “sympathy expressed by so many to my sister and brothers” had been the “greatest consolation”.
Watched by the Queen Consort, the new Prince of Wales and more than 200 privy counsellors – including six former prime ministers – the King pledged himself to the task now before him and the “heavy responsibilities of Sovereignty”.
The new monarch spoke after he had been proclaimed monarch, at 10am, to hold his first Privy Council meeting.
Watch ITV News' special coverage as Charles III is proclaimed monarch in a historic ceremony
The historic event comes after Charles gave a landmark address to the nation on Friday and paid a moving tribute to his “darling Mama” the Queen who died on Thursday afternoon at Balmoral.
Charles said of his “beloved mother” the late Elizabeth II: “We owe her the most heartfelt debt any family can owe to their mother; for her love, affection, guidance, understanding and example”.
ITV News UK Editor Paul Brand reports on the historical significance of Charles III being formally proclaimed king at the ceremony being held at St James's Palace
The King pledged his whole life as service as the new sovereign just as the Queen did, saying: “That promise of lifelong service I renew to you all today”.
The Proclamation ceremony featured a platform party, made up of Camilla, William, the Archbishop of Canterbury, the Lord Chancellor, the Archbishop of York, the Prime Minister, the Lord Privy Seal, the Lord Great Chamberlain, the Earl Marshal and the Lord President, who signed the Proclamation.
Lord President of the Council, Penny Mordaunt opened proceedings by announcing the death of the death of the sovereign and called upon the Clerk of the Council to read aloud the text of the Accession Proclamation.
It included Charles’s chosen title as King, already known to be King Charles III.
Making his declaration, the King said: “My lords, ladies and gentlemen, it is my most sorrowful duty to announce to you the death of my beloved mother the Queen.
“I know how deeply you, the entire nation, and I think I may say the whole world, sympathise with me in the irreparable loss we’ve all suffered.
“It is the greatest consolation to me to know the sympathy expressed by so many to my sister and brothers.
“And that such overwhelming affection and support should be extended to our whole family in our loss.”
He also signed a proclamation approving that the day of the Queen's State Funeral would be a Bank Holiday across England, Scotland, Wales and Northern Ireland.
On their return to Balmoral from a short prayer service, members of the royal family stepped out of their cars just before the bridge over the River Dee that leads towards the castle.
The group was led by the Queen's children, Prince Andrew, Prince Edward and Princess Anne.
Joined by Her Majesty's four granddaughters, the group stopped off to view the floral tributes that had been left behind at the gates of the Balmoral estate.
At 11am the crowds stood in a road overlooking the St James’s Palace’s Friary Court saw the spectacle of the Garter King of Arms reading the proclamation from a balcony.
In the court below was the palace’s detachment of the King’s Guard made up of Number 7 Company, Coldstream Guards.
The national anthem was performed by the Band of the Coldstream Guards alongside eight State Trumpeters of the Household Cavalry who were stood on the balcony.
Members of the Cabinet will also have their first meeting with the King just days after Liz Truss formed her new Government.
Numerous senior ministers attended the Accession Council at St James’s Palace where Charles was formally proclaimed sovereign.
A number of former prime ministers were also in attendance at the event, including Boris Johnson, Gordon Brown and Tony Blair.
Later they will travel to Buckingham Palace for an audience with King Charles III.
MPs finished the second of two days in the House of Commons on Saturday evening in which they paid tribute to the late Queen, closing by conveying their sympathy to the King for his loss.
Proceedings in the lower chamber began at 1pm with a small number of senior MPs taking an oath of allegiance to the King, and closed around 10pm.
Commons Leader Penny Mordaunt closed the second day of tributes, saying: “Our great Queen has entrusted us all with a living legacy of triumph over tribulation, of cheerfulness over challenge, of dedication and determination.
“She has left us. Her values remain with us. Her example compels us to continued fidelity to our King and our country. God save the King.”
A humble address was agreed unanimously by all those present, which conveyed “deep sympathy” to the King and his family for his loss, paid tribute to the late Queen for her “unstinting dedication”, and expressed the House’s “loyalty” to the King.
“This has been an amazing tribute,” Commons Speaker Sir Lindsay Hoyle said, adding “when the House comes together it comes together in a very special way, and there has been none more special than these last two days. 138 contributions today, making a total of 321 contributions in total.”
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