'Greatest sadness': UK's new King Charles III on Queen's death

Queen Elizabeth II's eldest son Charles III, the rightful heir to the British throne and new king, said on Thursday that the death of his mother is a moment of the "greatest sadness" for him and all members of his family.

Queen Elizabeth II, the United Kingdom's longest-serving monarch, died on Thursday at Balmoral Castle in Scotland after reigning for 70 years. She was 96.

Her death brings to an end the longest reign in the history of the United Kingdom, and one of the longest reigns by any head of state.

"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," said 73-year-old Charles, who is the oldest person to ever assume the British throne.

"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," the new king said in a statement.

During this period of mourning and change, he said his family and he will be comforted and sustained by knowledge of the respect and deep affection in which the queen was so widely held.