According to calculations by royal officials, at around 1630 GMT Queen Elizabeth will beat her great-great grandmother Victoria's time on the throne: a total of 63 years, seven months and two days which she served between 1837 and 1901.
The exact hour has been difficult to determine because the exact start of her reign -- the moment when her father George VI passed away -- is difficult to work out as the king died at night in his sleep.
According to reports, The Queen Elizabeth II is planning to keep things low-key on Wednesday when she will overtake Queen Victoria as Britain's longest-serving monarch, despite public interest in the historic date.
According to sources, the reason why the queen is not enthusiastic about the date is because the date whose calculation rests on the death of her father and great-great grandmother naturally colors the way she sees it,
"While she acknowledges it as an historic moment, it's also for her not a moment she would personally celebrate, which is why she has been keen to convey business as usual, and no fuss," the source said.
Buckingham Palace will mark the day with a photo display of her reign and the Royal Mint has designed a new silver £20 coin (27 euros, $30) with the five official portraits since she became queen in 1952.
The 89-year-old Elizabeth, also the world's oldest monarch, had originally not planned anything special for the day itself but reportedly agreed to a public appearance due to public pressure.
The queen will ride on a steam train in Scotland to inaugurate a new railway line and will host a dinner at Balmoral Castle with her grandson Prince William and his wife Kate in attendance.