The monarch had been placed under medical supervision at Balmoral Castle, her summer residence in Scotland, after doctors were "concerned" for her health, according to a statement shared by Buckingham Palace.
Her exact cause of death was not immediately shared.
Members of the royal family had rushed to be at her side after her health took a turn for the worst, including her 73-year-old son Prince Charles, who now automatically becomes king.
The announcement by the palace came a day after the queen canceled a virtual meeting of her Privy Council when doctors advised her to rest following a full day of events on Tuesday, when she formally asked Liz Truss to become Britain's prime minister.
"Following further evaluation this morning, the Queen’s doctors are concerned for Her Majesty’s health and have recommended she remain under medical supervision,'' a palace spokesperson said earlier Thursday. "The Queen remains comfortable and at Balmoral."
Media gather at the entrance to Balmoral in Scotland, where the Queen is under medical supervision with the royal family rushing to be by her side amid serious health fears. Picture date: Thursday, Sept. 8, 2022. (Photo by Andrew Milligan/PA Images via Getty Images)
The queen’s death came just three months after people across Britain paused over a long holiday weekend to celebrate her 70 years on the throne. While crowds of cheering, flag-waving fans filled the streets around Buckingham Palace throughout four days of festivities, the queen herself made only two brief appearances on the palace balcony to wave to her subjects.
Elizabeth had increasingly handed over duties to Charles and other members of the royal family in recent months as she recovered from a bout of COVID-19, began using a cane, and struggled to get around.
Since assuming the throne after the death of her father on Feb. 6, 1952, Elizabeth was a symbol of stability as Britain negotiated the end of empire, the dawn of the information age and the mass migration that transformed the country into a multicultural society.
That steadfastness was seen Tuesday when she oversaw the handover of power from Prime Minister Boris Johnson to Truss in a series of carefully choreographed events steeped in the traditions of Britain’s 1,000-year-old monarchy.
Throughout her tenure, the queen built a bond with the people of Great Britain through a seemingly endless series of public appearances as she opened libraries, dedicated hospitals and bestowed honors on deserving citizens.
But the death of her husband, Prince Philip, in April of last year reminded the country that the reign of the only monarch most people in Britain had ever known was finite.
Prince Charles, Prince of Wales, Queen Elizabeth II, Prince George of Cambridge, Prince William, Duke of Cambridge and Princess Charlotte of Cambridge stand on the balcony during the Platinum Pageant on June 05, 2022 in London. (Photo by Chris Jackso
During a funeral that was limited to just 30 mourners because of pandemic restrictions, the queen was forced to sit alone, hidden behind a black face mask, as she contemplated the loss of the man who had been at her side for more than 70 years.
That truth was the subtext of the Platinum Jubilee celebrations as newspapers, TV news shows and the walls of the palace were filled with images of Elizabeth as she evolved from a glamorous young queen in crown and diamonds to a kind of global grandmother known for her omnipresent handbag and love of horses and corgis.
Charles was front and center throughout the festivities as he stood in for his mother and demonstrated he was ready to take on her mantle.
Experts said Charles’ coronation might not take place for months. It was also not known whether he will call himself King Charles III or some other name.
The Associated Press contributed to this report. Editor's note: Pool video has since been removed from this story due to usage restrictions from the royal family.