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".
'Great fun'
Mr Cameron paid tribute to the Queen at a cabinet meeting on Monday, saying she had a "remarkable record" and was "a symbol of Britain's enduring spirit admired around the world".
Mary Francis, one of her aides in the late 1990s, said the very private monarch could also be "great fun".
"I've seen her on one of the last holidays on the Royal Yacht Britannia - the only time I ever saw the Queen wear trousers.
"She let her hair down. She told stories. She had everybody in fits of laughter so she can be very relaxed - at times."
Analysis
Getty ImagesBy BBC royal correspondent Nicholas Witchell
It will be day 23,226 of her reign. The Queen is determined that it should in no way be exceptional.
She has let it be known with some emphasis that she does not want a fuss to be made.
It is evidently viewed as bad form for one long-lived queen to be seen in any way to be celebrating the passing of a record set by another long-lived queen.
But of course that is precisely why 9 September 2015 is notable in its way. Elizabeth II will become the longest-reigning monarch in British history, passing the record set by her great-great-grandmother Victoria.
And in an institution as old as the monarchy, that is a rather striking measure to add to the other memorable features of her reign.
She may not want there to be a fuss but it would seem that a good number of British citizens, to say nothing of those from further afield, believe that her record-breaking reign deserves a little recognition.
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.
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.
Queens of the modern age
Getty Images- Victoria became queen at 18 while Elizabeth was 25
- Elizabeth II rides in the same coach as Victoria did for the annual State Opening of Parliament
- Both queens were shot at by a lone gunman while out riding near Buckingham Palace
- Elizabeth loves the private royal estate at Balmoral, which was bought by Victoria
- Victoria ruled over an empire of 400 million people. Elizabeth is head of state for 138 million people
Queen Victoria became queen at the age of 18 and ruled for 63 years, seven months and two days.
Queen Elizabeth's reign has included 12 prime ministers, two more than served under Victoria.
One of those prime ministers, Conservative Sir John Major, rejected any suggestion the Queen had been too passive as head of state: "The monarchy wouldn't be as popular if they were part of politics - they're above and beyond it.
"But when the Queen meets her prime minister she has the opportunity to question, to ask, to counsel. Nobody knows and no prime minister is going to tell you exactly what happens at those meetings. So those who say she's been too passive, how can they possibly know?"
The Queen is sovereign of 15 Commonwealth realms in addition to the UK, and Brian Mulroney, former prime minister of Canada, said she remained extremely popular there.
'Genuinely exceptional'
He also told BBC Radio 5live that while her position might appear only "symbolic", she had played an important role on the world stage, notably in the fight against apartheid in South Africa.
"She has her own way of conveying a view on an important national or international issue that, if you know her in any way, is unmistakeably clear.
"The fact that she has exercised authority in such an impeccable manner, her own reputation and her own personal conduct has been flawless... the only conclusion I can come to is that she is a genuinely exceptional person."
Buckingham Palace
Anti-monarchist group Republic said the Queen's long reign was a reason for reform not celebration.
Chief executive Graham Smith said: "It is now time for the country to look to the future and to choose a successor through free and fair elections, someone who can genuinely represent the nation."
