Prime Minister Liz Truss and other party leaders have taken the oath of allegiance to the new King as Parliament met for a rare Saturday sitting.
A select group of senior MPs were given the chance to formally pledge their loyalty to Charles at the Commons despatch box ahead of the second day of tributes to the Queen.
Commons Speaker Sir Lindsay Hoyle said: “Following the meeting of the Accession Council and the Principal Proclamation earlier today, I’ll first take the oath to His Majesty.
“Time constraints mean a small number of honourable members are able to take the oath and make the affirmation today.”
He added: “There will be further opportunities for all honourable members to take the oath or make the affirmation following her late majesty’s funeral.
“There is no procedural requirement to do so.”
MPs are sworn in after each general election so they can take their seat, speak in debates, vote and receive a salary.
The wording of the oath means MPs have already pledged their allegiance to the heirs and successors of the Queen, meaning they do not have to do it again at this point.
Sir Lindsay was the first MP to swear in followed by Father of the House Sir Peter Bottomley.
Mother of the House Harriet Harman was next in line before Ms Truss stepped forward.
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.”
Commons Leader Penny Mordaunt and Labour leader Sir Keir Starmer were among those MPs who opted to affirm.
SNP Westminster leader Ian Blackford, Liberal Democrat leader Sir Ed Davey, the DUP’s Sir Jeffrey Donaldson and Plaid Cymru Westminster leader Liz Saville Roberts were involved in the ceremony.
Ms Saville Roberts took the oath in both English and Welsh.
Conservative former prime minister Theresa May was also among the 30 MPs to take part in the 10-minute ceremony.
Sir Lindsay went on to confirm 182 MPs paid tribute to the Queen on Friday, with tens more MPs scheduled to do the same on Saturday.
Deputy Prime Minister and Health Secretary Therese Coffey praised the “constancy” the Queen gave the nation through decades.
Ms Coffey said wherever she travelled around the world during her time as an environment minister the Queen was “held in the highest regards and there were always representations made for her”.
Conservative former Cabinet minister Greg Clark said the Queen’s “dazzling 1,000-watt smile” for visitors showed she recognised everyone who met her would treasure the moment forever.
Mr Clark said he would “always be grateful” to former prime minister Boris Johnson for reappointing him to the Cabinet over the summer, noting: “I was able to swear the oath of office to Her Majesty in person herself on July 8 at Windsor Castle.
“And when I shook Her Majesty’s hand I was greeted with the most dazzling 1,000-watt smile, sparkly eyes that suggested she was absolutely thrilled to see me. Now, I strongly suspect that Her Majesty’s demeanour did not reflect the fulfilment of a three-year hope that the prime minister would restore me to office.
“Instead it showed that at the age of 96, on a hot summer’s afternoon, Her Majesty still recognised that for everyone she met it was a moment that they would treasure forever.”
Labour former minister Sir George Howarth said he became more supportive of the monarchy later in life, having never felt a strong connection with it in his youth.
The Knowsley MP said: “I have to confess that I have not always felt supportive of the principle of the monarchy. As a young local councillor I once attended a function at which the loyal toast to the Queen was proposed.
“Foolishly I declined to take part, remaining firmly in my seat. My non-participation led to comment in our local paper and a strong backlash from the people I represented.”
He added: “By the time I was elected to this House more than a decade later I had come to the view that Her Majesty and the monarchy were a much-valued part of our national life.”
