Charles, who became King after the death of his mother Queen Elizabeth II, said her passing is a "moment of the greatest sadness" for the royal family.
In a statement, he added he will be comforted by the "knowledge of the respect and deep affection" in which the late monarch was widely held.
Queen Elizabeth II, 96, was the nation’s longest-reigning monarch, serving as head of state for more than 70 years. She died peacefully on Thursday afternoon at Balmoral.
The Prince of Wales is now King, having acceded to the throne immediately on the death of his mother.
"The death of my beloved Mother, Her Majesty The Queen, is a moment of the greatest sadness for me and all members of my family," the King said in a statement issued by Buckingham Palace.
The statement continued: "We mourn profoundly the passing of a cherished Sovereign and a much-loved Mother. I know her loss will be deeply felt throughout the country, the Realms and the Commonwealth, and by countless people around the world.
"During this period of mourning and change, my family and I will be comforted and sustained by our knowledge of the respect and deep affection in which The Queen was so widely held."
You can hear more tributes to Her Majesty The Queen following her death at 96