Queen Elizabeth II becomes Britain's longest-reigning monarch later when she passes the record set by her great-great-grandmother Queen Victoria.
The Queen will have reigned for 63 years and seven months - calculated at 23,226 days, 16 hours and approximately 30 minutes at about 17:30 BST.
Prime Minister David Cameron will lead tributes in the House of Commons and there will be a River Thames salute.
The Queen, who is 89, will spend the day on official duties in Scotland.
The exact moment the Queen reaches the milestone is not known because her father, George VI, passed away in the early hours of 6 February 1952.
Business in the Commons will be postponed for half an hour so MPs can pay tribute to the Queen.
On the Thames, a flotilla of historic vessels, leisure cruisers and passenger boats will take part in a procession between Tower Bridge and the Houses of Parliament.
The bridge will lift as a sign of respect and HMS Belfast will sound a four-gun salute.
The BT Tower in central London will scroll the message "Long may she reign".
Buckingham Palace has released an official photograph to mark the occasion, taken by Mary McCartney in the Queen's private audience room.
This is where she holds weekly audiences with prime ministers of the day, and receives visiting heads of state and government.
The prime minister's official spokeswoman said Mr Cameron had paid tribute to the Queen at a cabinet meeting on Monday.
Mr Cameron said the Queen had a "remarkable record" and was "a symbol of Britain's enduring spirit admired around the world".
It is understood that Wednesday will be a normal working day for the monarch with no special celebration.
She will be joined by the Duke of Edinburgh to open the £294m Scottish Borders Railway and make a steam train journey with Scotland's First Minister Nicola Sturgeon.
The Queen is taking her traditional summer break at this time of year at her private Scottish home, Balmoral.