The Queen will soon make history when she becomes the longest reigning British monarch.
On September 9 she will pass the record set by her great-great-grandmother Queen Victoria who, Buckingham Palace has calculated, reigned for 23,226 days, 16 hours and 23 minutes.
Historian Dr Kate Williams described the length of the Queen's reign as a great source of strength for the monarch.
"It's a very significant milestone. The Queen will become the longest reigning British monarch in history. She came to the throne when she was older than Victoria. Victoria was only 18," Dr Williams said.
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"It shows that our female monarchs last the longest. The Queen's longevity is a great source of her strength and popularity. She has lived through World War Two and throughout the 20th century. Many people will not have known a different monarch."
Queen Victoria, who died in 1901, acceded to the throne on June 20 1837 and became the figurehead of a vast empire amid great industrial, cultural and scientific changes to society.
Princess Elizabeth became Queen Elizabeth II when she was 25 on the death of her father King George VI on February 6, 1952. Like Victoria, she has acted as a figure of continuity as the country has modernised.
She has served, with Prince Philip at her side, through the 20th century, the Millennium and into the 21st century, witnessing new technological advances and a succession of British governments of different political persuasions.
The Queen passes Victoria's record at around 5.30pm on September 9 when she will have reigned for 23,226 days 16 hours and approximately 30 minutes - but the exact moment she became Queen is hard to calculate as George VI died in his sleep in the early hours of the morning, possibly at around 1am.