"The Queen died peacefully at Balmoral this afternoon. The King and the Queen Consort will remain at Balmoral this evening and will return to London tomorrow," Buckingham Palace said on Thursday.
The Queen's eldest son and heir, Charles, the former Prince of Wales, has become King of the United Kingdom and 14 Commonwealth realms.
In a statement, he 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. 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."
Queen Elizabeth II was the longest-reigning monarch in British history and at 96 was the world's oldest head of state.