The King has held his first in-person audience with the new Prime Minister since he ascended to the throne.
The 73-year-old succeeded his mother following her death yesterday afternoon, having appointed Liz Truss as the 15th PM of her reign just two days earlier.
Today MPs have paid emotional tributes to the UK's longest-serving monarch in history, who took the throne at the age of 25 and was head of state until she was 96.
Ms Truss led the tributes, telling a special session in the House of Commons that the Queen was "the rock on which modern Britain was built".
She said: "Her late majesty, Queen Elizabeth II, was one of the greatest leader the world has ever known."
You can leave your tributes to Queen Elizabeth II here
In her speech to the Commons, the Prime Minister voiced her support for the new king.
Ms Truss said: "His Majesty King Charles III bears an awesome responsibility that he now carries for all of us.
"I was grateful to speak to His Majesty last night and offer my condolences. Even as he mourns, his sense of duty and service is clear.
"He has already made a profound contribution through his work on conservation, education and his tireless diplomacy.
"We owe him our loyalty and devotion."
She continued: "All of us in this House will support him as he takes our country forward to a new era of hope and progress. Our new Carolean age.
"The crown endures. Our nation endures. And in that spirit, I say God save the King."
Earlier the King met crowds outside Buckingham Palace after travelling to London from Balmoral, where the Queen was staying when she died.
The palace said on Friday morning: “Following the death of Her Majesty the Queen, it is His Majesty the King’s wish that a period of royal mourning be observed from now until seven days after the Queen’s funeral.
“The date of the funeral will be confirmed in due course.”
The first official rendition of God Save the King will be sung at St Paul's Cathedral at the end of a memorial service for the Queen.
The lyrics to the national anthem will change from "Queen" to "King" and "her victorious" to "him victorious" to mark that King Charles III has now taken over as monarch.
It comes after crowds spontaneously sang the version of the song outside of Buckingham Palace on Friday as the King arrived with the Queen.
* 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 both pullouts.
