Queen Elizabeth II, the UK's longest-serving monarch, has died at Balmoral aged 96, after reigning for 70 years.
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.
With her death, 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."
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."
At Buckingham Palace in London, crowds awaiting updates on the Queen's condition began crying as they heard of her death. The Union flag on top of the palace was lowered to half-mast at 18:30 BST.
Prime Minister Liz Truss, who was appointed by the Queen on Tuesday, said the monarch "provided us with the stability and strength that we needed".She said His Majesty would be known as King Charles III.