STORY: The new prime minister's first meeting with the new monarch came after Charles returned to London from Scotland.
"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 well-wishers outside the palace.
"It has been so touching this afternoon when we arrived, all those people who had come to give their condolences ... and flowers," Charles said to Truss, as the prime minister also offered her sympathies.
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.