Senior MPs including Prime Minister Liz Truss have sworn a new oath of allegiance to King Charles III in a special session of Parliament, BBC reports.
Commons Speaker Sir Lindsay Hoyle opened proceedings by taking the oath, followed by Sir Peter Bottomley, the longest-serving MP.
Sir Keir Starmer and former PM Theresa May were also pledging allegiance.
In a rare Saturday sitting, MPs and peers are gathering for a second day of tributes to the late Queen.
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 told MPs: "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."
Ms Truss was foremost among those taking the pledge, speaking the words: "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."
From Saturday night, Parliament will be postponed until a period of national mourning has ended.
Ms Truss, who was invited to form a government by the late Queen on Tuesday, had her initial audience with King Charles on Friday.
The prime minister curtsied as she was ushered in by a royal aide for the first of what will be their regular weekly encounters.
