King Charles III has been formally proclaimed monarch at the Accession Council - and has used a new signature for the first time. During the first part of the meeting, His Majesty's role as King and regal name were confirmed in front of many important attendees, including Queen Consort Camilla, heir to the throne Prince William and several former Prime Ministers.
The event was televised for the first time ever, with Queen Elizabeth II's happening behind closed doors 70 years ago, which gave us incredible insight into the formal process. The new King followed a tradition going back centuries by signing the declaration with 'Charles R' - but he added the letter for a different reason to his late mother.
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The Queen used the signature 'Elizabeth R' on official documents - but it's not to represent a secret name.
It actually stands for 'regina', which means 'queen' in Latin - and because Charles is King, his 'R' stands for 'rex'.
The tradition of signing with the initial R goes back centuries for English monarchs and is used to designater authority of the sovereign.
It goes back as far as Henry I, who ruled in the early 12th century, and is how all official documents and formal letters are signed.
There have been exceptions, as Queen Victoria used 'Victoria RI', which stands for Rex Imperator, when she became Empress of India.
Although it's likely that Charles won't add the 'R' when he's writing to family and friends, as his late mother used her nickname 'Lilibet'.
Royal spouces can also incorporate the 'R' into their signatures, with Queen Elizabeth the Queen Mother using the regal initial.
However, Prince Philip only used his first name when he put pen to paper.
When William takes to the throne, he will also use the initial and sign as 'William R'.
More than 200 privy councillors, made up of mostly senior politicians past and present including former prime ministers, were present to hear the Clerk of the Council read the proclamation.
He made a personal declaration on the death of the Queen saying: "I know how deeply you and the entire nation, and I think I may say the whole world, sympathise with me in this irreparable loss we have all suffered.
"Her reign was unequalled in its duration, its dedication and its devotion. Even as we grieve, we give thanks for this most faithful life."
A wave of national transformation is now in full swing as once-in-a-generation procedures that have been decades in the planning are put into action. His Majesty will be meeting many powerful figures today as he is holding audiences at Buckingham Palace.
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The new King has a busy few days ahead, as he is now preparing to embark on a tour of the country in the days before the funeral to meet his new subjects as millions around Britain unite in their sorrow.
On Monday, Charles will receive the motion of condolence at Westminster Hall before starting his UK tour with a visit to the Scottish parliament and a service at St Giles Cathedral in Edinburgh.
Tuesday will see the King arrive in Northern Ireland, where he will receive another motion of condolence at Hillsborough Castle and attend a service at St Anne's Cathedral in Belfast.
He will then travel to Wales on Friday to receive another motion of condolence at the Welsh parliament and attend a service at Llandaff Cathedral in Cardiff.
This comes after King Charles addressed the nation for the first time in a pre-recorded video message on Friday evening.
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The new monarch covered a range of topics during his six-minute speech, which was incredibly well received by the public and left some in tears.
The King announced that William and his wife, Kate Middleton, would become the new Prince and Princess of Wales, as well as praising his wife and new Queen Consort Camilla.
He also sent love to his youngest son and his wife Meghan Markle, saying: "I want also to express my love for Harry and Meghan as they continue to build their lives overseas."
The biggest tribute was obviously for his "darling mama" and his late father Prince Philip.
Charles ended his speech by saying: "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'."
* This weekend, the Daily Mirror and Sunday Mirror celebrate the life of Her Majesty the Queen with a commemorative special filled with all the key moments from Britain’s longest reigning monarch. Be sure to pick up your copy of the Daily Mirror and Sunday Mirror to get poth pullouts.
You can leave your tributes to Queen Elizabeth II here