The Queen will become Britain's longest reigning monarch today - and tributes are expected to flood in to mark the historic occasion.
The milestone is likely to set the seal on her reign as sovereign, which so far has lasted more than 63 years.
There is speculation the Queen may reflect on the historic day that sees her eclipse her great-great-grandmother Queen Victoria's reign, if she gives a speech when she opens the new £294 million Scottish Borders Railway.
The day will begin with the Queen and Duke of Edinburgh arriving at Waverley Station in Edinburgh to board a train with Scotland's First Minister Nicola Sturgeon. The train will be drawn by the steam locomotive Union of South Africa.
The royal party will travel the new route, stopping off at Newtongrange in Midlothian before carrying on to the final station on the railway, the Borders town of Tweedbank, where the opening ceremony will be held.
Celebrity photographer Mary McCartney, who has captured the Queen at work to mark the historic day, has described her as a "beacon for womankind''.
The official photograph released today and taken by McCartney, the daughter of former Beatle Sir Paul McCartney, shows the Queen sitting at her desk in Buckingham Palace, working on matters of state.
McCartney said: "Having grown up during the reign of Queen Elizabeth II it was a thrill to meet her and a very great privilege to take her photo on this historic occasion.
"She is a truly inspirational person, a trailblazer and a beacon for womankind.''
David Cameron has already paid tribute to the Queen, telling Cabinet colleagues at their weekly meeting in Downing Street yesterday that the Queen had a "remarkable record'' and was "a symbol of Britain's enduring spirit admired around the world".
Later today, the Queen will pass the record set by Queen Victoria, who defined an era and left an indelible mark on 19th century British history.
The exact moment the Queen passes Victoria's milestone is not known as there is no precise time for the death of her father, George VI - who died in the early hours of February 6 1952.
But it is thought he probably died at 1am, so at around 5.30pm the Queen will make history as Buckingham Palace has calculated she will have reigned for 23,226 days, 16 hours and approximately 30 minutes, a few minutes longer than Victoria.
Celebrations will be held throughout the day, with church bells ringing out across the country and business in the House of Commons postponed for half an hour so MPs can pay tribute to the Queen.
The BT Tower will scroll the message ''Long May She Reign'' and the royal rowbarge Gloriana will join a flotilla of boats in a procession down the River Thames.
Tower Bridge will lift as a mark of respect and, as the procession passes HMS Belfast, a four-gun salute will sound out and the Massey Shaw fireboat will shoot jets of water into the air.