The exact moment at which the 89-year-old monarch will reach the milestone is hard to pinpoint, as the precise time of her father king George VI’s death in his sleep in 1952 is not known.
However, the queen will not indulge in fanfare or celebrations, marking the occasion with just another public engagement — this time opening a new railway line in Scotland. Nor are there widespread street parties or other official events planned.
The mood is in keeping with the sober style of her reign, something inherited from her father and grandfather King George V.
The queen has “established a record of unimpeachable integrity”, said historian David Starkey.
“I think she is immensely respected. She has played the role with a certain dignity. In this touchy-feely age she has performed… in a slightly old-fashioned way.”
While queen Victoria, who reigned for 63 years and 216 days between 1837 and 1901, saw Britain’s power reach its zenith with a global empire and vast industrial expansion, the current queen’s rule has largely been one of British contraction.
It has seen Britain rebuild following World War II, dismantle its empire and turn to Europe.
The kingdom has weathered economic collapse in the 1970s and revival in the 1980s, unrest in Northern Ireland, the emergence of a less deferential society and mass immigration changing the country’s make-up.
The queen also calmly steered the monarchy out of the scandals that engulfed her reign in the 1990s.
She described 1992, in which her sons Prince Charles and Andrew separated from their wives and daughter Princess Anne divorced her husband, as her “annus horribilis”.
Her popularity reached its nadir when the “people’s princess” Diana, by then divorced from her son and heir Prince Charles, was killed in a car crash in Paris in 1997.
Britain was plunged into an unprecedented outpouring of public grief and the monarch was accused of lacking compassion and misreading the national mood in the aftermath.
Her decision to keep the family — including Diana’s sons, Princes William and Harry — mourning privately at their estate in Scotland rather than returning to London was deeply unpopular, as was the failure to lower the royal flag as Buckingham Palace.
After days of intense pressure, she returned to London and addressed the nation on TV, beginning a slow rehabilitation of the monarchy that saw her golden jubilee warmly celebrated by the public in 2002.