Queen Elizabeth II, the UK's longest-serving monarch, has died at Balmoral aged 96, after reigning for 70 years.
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Her family 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.
Prime Minister Liz Truss said the Queen was the rock on which modern Britain was built, and the new King would be known as King Charles III.
In a statement, His Majesty the King, who also becomes head of state for 14 Commonwealth realms, 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."
He said during the period of mourning and change he and his family would be "comforted and sustained by our knowledge of the respect and deep affection in which The Queen was so widely held".
In a statement, Buckingham Palace said: "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."
All the Queen's children travelled to Balmoral, near Aberdeen, after doctors placed the Queen under medical supervision.
Her grandson, Prince William, is also there, with his brother, Prince Harry, on his way.
Elizabeth Alexandra Mary Windsor was born on 21 April 1926, in a house just off Berkeley Square in London, the first child of Albert, Duke of York, second son of George V, and his duchess, the former Lady Elizabeth Bowes-Lyon.
On 20 November 1947 Elizabeth married Prince Philip of Greece in Westminster Abbey.
The Duke of Edinburgh, as Philip had become, remained a serving naval officer.
In January 1952, Elizabeth, then 25, set off with Philip for an overseas tour. The King, against medical advice, went to the airport to see the couple off. It was to be the last time Elizabeth would see her father.
Elizabeth heard of the death of the King while staying at a game lodge in Kenya and the new Queen immediately returned to London.
Her Coronation in June 1953 was televised, despite the opposition of Prime Minister Winston Churchill, and millions gathered around TV sets, many of them for the first time, to watch as Queen Elizabeth II made her oath. ■