Prince William's sweet gesture of support to Queen Camilla as Charles III is declared King

King Charles III has now been formally declared the King of the United Kingdom and the Commonwealth.

The monarch was joined by his wife Camilla, Queen Consort and son Prince William in the trio's first official public appearance since Queen Elizabeth II's death.

The former Prince of Wales was formally proclaimed King in front of the Accession Council – comprised of Privy Counsellors – at approximately 7pm AEST at St James's Palace in London.

Catherine, the Princess of Wales, was noticeably absent, as was the King's second son Prince Harry – though protocol determined they did not attend.

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Prince William walked behind a sombre Queen Camilla, patting his step-mother on the back reassuringly as they entered a room full of Cabinet ministers, the Archbishops of Canterbury and York, the Speaker of the House of Commons, the First Ministers of Scotland, Wales and Northern Ireland, and the Lord Chief Justice of England and Wales.

King Charles did not enter the room where the proclamation was signed, first by Prince William and then Queen Camilla, who reportedly signed the document "Camilla R" to signify her new status, "Regina."

Following Queen Camilla's signing of the proclamation of King Charles as sovereign, the British Prime Minister Liz Truss and the Archbishops of Canterbury and York signed as well, as did the rest of the Judicial Council.

Watch the video above.

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Queen Camilla, Prince William and Truss were the first members of the Privy Council to meet with King Charles III in the Council Chamber where he was waiting as the proclamation was signed, as the rest of the Privy Council shuffled into the Throne Room of St James's Palace.

Truss was the first member of the Judicial Council to enter the Throne Room from the Council Chamber, with Queen Camilla entering before Prince William, and Prince William entering before King Charles, who entered last.

King Charles III then addressed the Privy Council, including a touching nod to his wife Queen Camilla for her support, and a tribute to his late mother, Queen Elizabeth II.

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Privy Counsellors, who are summoned to the Accession Council to formally oversee the proclamation of a new sovereign, are appointed for life by the monarch, who is advised by the Prime Minister.

Other members of the Privy Council include select senior royal family members – in this case, Prince William and Queen Camilla – as well as senior judges, leaders of Opposition parties, and leading Commonwealth spokesmen.

Prince Harry and the Princess of Wales are not members of the Privy Council.

The Privy Council was originally the executive arm of the English Government, though in modern times, it acts as an advisory body to the monarch.

The full Privy Council currently comprises of 718 members, though not all are required at all times – large meetings of the Privy Council are extremely rare, and are known as the Accession Council, who gathered today.

The Accession Council only convenes to proclaim a new monarch, or when the reigning sovereign announces their intention to marry.