"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," said the new king, her eldest son Charles, 73.
"I know her loss will be deeply felt throughout the country, the realms and the Commonwealth, and by countless people around the world."
News that the Queen's health was deteriorating emerged after noon on Thursday when her doctors said she was under medical supervision, prompting her family to rush to Balmoral Castle in Scotland to be by her side.
Thousands gathered outside Buckingham Palace in London, and there was a stunned silence when the flag was lowered to half-mast. The crowd surged to the gates as the death notice of the only monarch most Britons have known was attached to the iron gates.
King Charles III and his wife Camilla, the Queen Consort, will remain at Balmoral before returning to London on Friday, when Charles is expected to address the nation and meet Prime Minister Liz Truss. Details of the funeral have not been confirmed.
On Elizabeth's death, Charles automatically became monarch of the United Kingdom and the head of state of 14 other realms including Australia, Canada and New Zealand.
The Queen, whose husband died last year, had been suffering from what Buckingham Palace called "episodic mobility problems" since the end of last year, forcing her to withdraw from nearly all her public engagements.
Her last official duty was on Tuesday, when she appointed Truss prime minister, the 15th of her reign.
"Through thick and thin, Queen Elizabeth II provided us with the stability and the strength that we needed. She was the very spirit of Great Britain, and that spirit will endure," Truss said outside her Downing Street office.
Condolences poured in from leaders around the globe.
"Her legacy will loom large in the pages of British history, and in the story of our world," US President Joe Biden said in a statement.
In Paris, the mayor announced the lights of the Eiffel Tower would be turned off; in Brazil, the Christ the Redeemer statue was illuminated with the colours of the Union Jack; and the United Nations General Assembly and the Security Council stood for a moment of silence.
Even Russian President Vladimir Putin, whose country's relations with Britain have plummeted over the war in Ukraine, called it an "irreparable loss".
Queen Elizabeth II, who was also the world's oldest and longest-serving head of state, came to the throne following the death of her father King George VI on February 6, 1952, when she was just 25.
She was crowned in June the following year. The first televised coronation was a foretaste of a new world in which royal lives were to become increasingly scrutinised by the media.
"I have in sincerity pledged myself to your service, as so many of you are pledged to mine. Throughout all my life and with all my heart I shall strive to be worthy of your trust," she said in a speech to her subjects on her coronation day.
Elizabeth became monarch when Britain still retained much of its old empire. It was emerging from the ravages of World War II, with food rationing still in force and class and privilege still dominant in society.
In the following decades, Elizabeth witnessed massive political change and social upheaval at home and abroad. Her own family's tribulations, most notably the divorce of Charles and his late first wife Diana, were played out in full public glare.
While she remained a symbol of stability and continuity for Britons, her grandson Prince William told a 2012 documentary the Queen had "managed to modernise and evolve the monarchy like no other".
Her long reign meant she repeatedly broke records for British rulers. When she surpassed the more than 63 years her great-great-grandmother Queen Victoria spent on the throne, she said it was not a landmark to which she had ever aspired.
Her marriage to Prince Philip lasted 73 years, until his death in April 2021, and they had four children, Charles, Anne, Andrew and Edward.
She never gave a media interview and critics said she came across as distant and aloof.
But for the vast majority of her subjects she was a figure who commanded respect and admiration. Her death marks the end of an era.
Opinion polls suggest Charles does not enjoy anywhere near the same level of support and there is speculation the loss of Elizabeth might lead to a rise in republican sentiment, particularly in the other realms.
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