Liz Truss and other senior ministers swore an oath to King Charles in the House of Commons today.
After today, Parliament will shut down for a period of mourning following the death of Queen Elizabeth II.
But it has remained open for two days to hear tributes from MPs and peers.
Before that, cabinet ministers and senior MPs lined up to swear an oath to the monarch - something that normally happens after a general election.
Only some MPs chose to swear the new oath - which is not a formal requirement, as the oath sworn to Her Late Majesty also included “her heirs and successors”.
At the opening of the Commons this afternoon, Speaker Lindsay Hoyle told MPs there would be another opportunity for MPs to take the oath after the Queen's funeral.
He was first to take the oath, followed by Father of the House, Sir Peter Bottomley and Mother of the House, Harriet Harman.
Ms Truss followed, with Penny Mordaunt, Chief Whip Wendy Morton, Scotland Secretary Alistair Jack and Northern Ireland Secretary Chris Heaton Harris close behind.
Labour leader Keir Starmer was next, followed by Shadow Leader of the Commons Thangam Debbonaire and Sir Alan Campbell, the Labour chief whip.
The PM and her cabinet will later they will travel to Buckingham Palace for an audience with the monarch.
Ms Truss, who was invited to form a Government by the late Queen on Tuesday, had her initial audience with the King on Friday.
The Prime Minister curtsied as she was ushered in by a Palace aide for the first of what will be their regular weekly encounters.
As she offered her condolences, the King shook her hand and thanked her, saying: "You are very kind for coming - I know how busy you are."
He added: "But it's been so touching this afternoon when we arrived here, all those people come to give their condolences."
Ms Truss again repeated: "Your Majesty, my very greatest sympathies."
He replied: "You are very kind. It was the moment I have been dreading, as I know a lot of people have. We'll try to keep everything going. Come, come have a seat."
Earlier, the Prime Minister had led tributes in the House of Commons to the Queen as she urged the country to support their new monarch.
Those tributes will continue during rare Saturday sittings of both the Commons and the House of Lords.
