Queen Elizabeth II, the UK's longest-serving monarch, has died at Balmoral aged 96.
The Queen reigned for 70 years.
Her eldest son Charles, the former Prince of Wales, will lead the country in mourning as the new King and head of state for 14 Commonwealth realms.
In a statement, His Majesty The King said: "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.
"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."
Her family had gathered at her Scottish estate after concerns grew about her health earlier on Thursday.
The Queen came to the throne in 1952 and witnessed enormous social change on a global scale.