The Queen is to preparing to reflect on becoming Britain's longest reigning monarch amid celebrations for the historic milestone.
As she opens the new £294 million Scottish Borders Railway, the Queen is to give a speech in which she is expected to make an oblique reference to the landmark event which sees her eclipse the record of her great-great-grandmother Queen Victoria.
The Queen will arrive with the Duke of Edinburgh at Waverley Station in the Scottish capital 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.
David Cameron - the 12th prime minister of the Queen's reign - paid tribute, saying: ''Over the last 63 years, Her Majesty has been a rock of stability in a world of constant change.''
Celebrity photographer Mary McCartney, who has captured the Queen at work to mark the historic day in a new official photograph, has described her as a "beacon for womankind''.
The picture 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.
She 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.''
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.
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 about 30 minutes, a few minutes longer than Victoria.
Celebrations are being 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.
From sunrise, the BT Tower in central London has been scrolling the message ''Long May She Reign'' and later 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 and the Massey Shaw fireboat will shoot jets of water into the air.