The prime minister’s first meeting with the new monarch came after Charles returned from Scotland to London to cheering crowds outside Buckingham Palace who had come to pay their respects to the queen.
“The moment I’ve been dreading, as I know a lot of people have,” Charles was heard saying to the prime minister as they met in the audience room at Buckingham Palace.
Earlier, Charles and his wife, Queen Consort Camilla, spent more than 10 minutes shaking hands with dozens of well-wishers and looking at floral tributes outside the palace after getting out of their car for an impromptu walkabout.
“We mustn’t take up too much of your time,” Charles said to Ms Truss, who only took office herself 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, as the prime minister also offered her condolences.
The meeting took place ahead of a televised address by Charles to the nation, in which he pledged to follow the example of his mother in devoting himself to duty.
In his first speech as monarch, King Charles spoke of his “profound sorrow” over the death of his mother, calling her “an inspiration and example to me and to all my family”.
“That promise of lifelong service I renew to all today,” he said in the nine-and-a-half minute address, recorded earlier in the day and delivered with a framed photo of the Queen on a desk in front of him.
“As the Queen herself did with such unswerving devotion, I, too, now solemnly pledge myself, throughout the remaining time God grants me, to uphold the constitutional principles at the heart of our nation,” he said.
The King’s speech was broadcast on television and streamed at St Paul’s Cathedral, where some 2,000 people attended a service of remembrance.