The Queen has been hailed as a "beacon of womankind" and a "rock of stability" as tributes poured in on the day she is due to become Britain's longest-reigning monarch.
Although the exact time she will achieve the record is not known, Buckingham Palace estimates that Her Majesty will surpass her great-great-grandmother Queen Victoria by a few minutes at 5.30pm, having reigned for approximately 23,226 days, 16 hours and 30 minutes.
To mark the occasion, a new picture taken by celebrity photographer Mary McCartney - the daughter of former Beatle Sir Paul - has been released.
It shows the head of state sitting at her desk in Buckingham Palace undertaking official business.
Upon the image's release, 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."
Also speaking ahead of the day's celebrations, David Cameron paid tribute to the monarch for her "extraordinary record" in addition to her "grace and dignity".
- Queen not planning any events to mark occasion
There had been speculation that a celebratory private event was planned for today.
However, in keeping with the Queen's business-as-usual approach, Wednesday will be a normal working day for the monarch with no special dinner party, despite the nationwide fanfare.
The Queen is usually taking her traditional summer break at her private Scottish home of Balmoral at this time of year, but will open the new Scottish Borders railway and take a stream train ride with First Minister Nicola Sturgeon to mark its opening.
- Celebrations to take place across the country
While the Queen herself will not be celebrating, tributes will be paid to the monarch in various ways throughout the day.
Bells will ring out at churches across Britain and business in the House of Commons will be postponed for half an hour so MPs can pay tribute to the Queen.
The BT Tower will scroll a message reading "Long May She Reign", while 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.