Buckingham Palace announced her death in a short statement, triggering 10 days of national mourning and an outpouring of tributes to her long life and record-breaking reign.
"The Queen died peacefully at Balmoral this afternoon," said the statement issued at 1730 GMT.
"The King (Charles) and The Queen Consort (Camilla) will remain at Balmoral this evening and will return to London tomorrow."
The eldest of her four children, Charles, Prince of Wales, who at 73 is the oldest heir apparent in British history, becomes king immediately.
Royal officials confirmed he will be known as King Charles III.
From the steps of 10 Downing Street, Prime Minister Liz Truss mourned "the passing of the second Elizabethan age" nearly 500 years after the first and concluded: "God save the king."
Charles himself called his mother's death "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," he added in a statement signed "His Majesty the King".
The Queen's death came after the palace announced earlier yesterday that doctors were "concerned" for her health and recommended she stay under medical supervision.
All her children -- Charles, Princess Anne, 72, Prince Andrew, 62, and Prince Edward, 58, flocked to her Scottish Highland retreat, Balmoral.
They were joined by Charles's elder son, Prince William, and William's estranged brother Prince Harry.
Just two days earlier, the Queen performed one of her core ceremonial functions as head of state, appointing Truss as the 15th prime minister of her reign, which started with Winston Churchill in Downing Street.
She was seen smiling in photographs but looking frail and using a walking stick.
One photograph of the meeting with Truss sparked alarm, showing a deep purple bruise on the monarch's right hand.
Queen Elizabeth II came to the throne aged just 25 in 1952 in the exhausted aftermath of World War II, joining a world stage dominated by political figures from China's Mao Zedong to Soviet leader Joseph Stalin and US president Harry S. Truman.
Her 70-year reign straddled two centuries of seismic social, political and technological upheaval.
But throughout, she remained consistently popular and was Queen and head of state not just of the United Kingdom but 14 former British colonies, including Australia and Canada. New Zealand proclaimed Charles its new king.
She was also head of the 56-nation Commonwealth, which takes in a quarter of humanity, and supreme governor of the Church of England, the mother church of the worldwide Anglican communion.
"I think she'll be remembered as the greatest monarch in history, the longest serving but also the greatest," royal author Phil Dampier told AFP.
The national mourning will culminate in a final public farewell at Westminster Abbey in London.
Charles' coronation, an elaborate ritual steeped in tradition and history, will take place in the same historic surroundings, as it has for centuries, on a date to be fixed.
The mother of one of the most famous families in the world, she retained huge public support throughout, surviving even a backlash in the wake of the shocking death of Charles's first wife, Diana, in 1997.