The King told Liz Truss the death of the Queen was a moment he had “been dreading”, as he held his first audience with the prime minister at Buckingham Palace.
King Charles III shook Ms Truss’s hand as he welcomed her to the first of what will be their weekly meetings.
Discussions between the monarch and PM are traditionally private, but cameras caught the opening moments of their Friday afternoon meeting.
“The moment I’ve been dreading, as I know a lot of people have,” King Charles told Ms Truss of his mother’s death.
“We mustn’t take up too much of your time,” he remarked to the PM, who only took on her job on Tuesday.
“It has been so touching this afternoon when we arrived, all those people who had come to give their condolences … and flowers,” he added.
Ms Truss also offered her condolences.
The King travelled back from Balmoral today after rushing there when the Queen‘s health deteriorated.
He became King when the Queen died yesterday afternoon.
Ms Truss became prime minister on Tuesday, a day after winning the Conservative Party leadership contest, and travelled to Balmoral that morning to meet the Queen, who then asked her to form a government.
The picture of the two meeting showed the Queen’s last public appearance.
Just 48 hours later, Ms Truss gave a speech on the steps of Downing Street to mark the Queen’s death and offered her support to the King.
Her new cabinet met this morning to pay tribute to the Queen, and MPs and peers remembered her in a special session of parliament.
She was also pictured signing a book of condolence in 10 Downing Street.
The King’s first meeting with Ms Truss came ahead of his first televised address to the nation.
Earlier, King Charles greeted well-wishers outside Buckingham Palace.
It was his first walkabout and encounter with the public since his accession.
One woman leaned over to kiss his right hand and another kissed him on the cheek as he thanked people for their good wishes and shook hands with them.
Tomorrow morning, Ms Truss will attend the Accession Council in the State Apartments of St James’s Palace, where King Charles will be formally proclaimed King. It will be the first time in history the process has been televised.