The Queen has been described as a "beacon for womankind" by celebrity photographer Mary McCartney who has captured the Queen at work to mark the moment she becomes the longest reigning monarch in British history.
She will pass the record set by her great-great-grandmother 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.
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.
The photographer approached the Royal Household with the proposal of taking the Queen's portrait to mark the milestone and her project was accepted.
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", said the PM's official spokeswoman.
His words was greeted by supportive banging of the Cabinet table by ministers.
There had been speculation that a celebratory private event was planned for today. But it is understood that, 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.
At this time of year, the Queen is taking her traditional summer break at her private Scottish home of Balmoral.
But on the day she passes Victoria's milestone, the Queen - joined by the Duke of Edinburgh - will open the new Scottish Borders Railway and take a steam train ride on the new £294 million railway with Scotland's First Minister Nicola Sturgeon.
Members of the Royal Family including the Prince of Wales and Duchess of Cornwall will also be working.
The commemorative photograph is not a formal one, but an image showing the Queen working from papers delivered to her in a famous red box - which is prominent in the picture's foreground.
Every day of the year – except Christmas Day – wherever she is, the Queen receives from government ministers, and from her representatives in Commonwealth and foreign countries, information in the form of policy papers, Cabinet documents, Foreign Office telegrams, a daily summary of events in Parliament, letters and other State papers.
These are sent up to her by the private secretaries in red boxes also used by Government ministers to carry confidential documents.
All of the papers have to be read and, where necessary, approved and signed.
They are made by leather goods company Barrow and Gale and the Queen still uses the boxes made for her on her coronation. They have been refurbished over the years and while Government boxes bear the Royal cypher, only the monarch's box is embossed with the words "The Queen".
McCartney has followed in the footsteps of her photographer mother Linda McCartney and is now an in-demand photographer who has taken the pictures of a host of famous names from Gwyneth Paltrow to Jude Law.
In the image, taken in July, the Queen is wearing a pink and white floral print day dress by designer Karl Ludwig, also worn by the monarch in Perth on the last day of her tour of Australia in 2011, and a pink sapphire brooch surrounded by diamonds.
She is clasping a pair of spectacles in her right hand as she holds a piece of paper and resting against the back of her chair is her black handbag.
Family pictures line a nearby sideboard in the room which the Queen uses for her private weekly audiences with the Prime Minister and to meet visiting heads of state like President Barack Obama.
The ornate desk in the picture was featured in the opening ceremony of the London 2012 Olympics when the Queen starred alongside James Bond, played by Daniel Craig.
During the famous footage, 007 is ushered into a Buckingham Palace room where the Queen is quietly working at the desk decorated with family portraits that feature in the McCartney photograph - separate images of her parents George VI and the Queen Mother and another of her grandson the Duke of Cambridge.
Celebrations will be held throughout the day, with church bells ringing out across the country 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 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.
ends
The Queen demonstrated her business as usual approach at the end of her short speech by remarking: "So now to the business in hand. It is my very happy duty to declare the Borders Railway open."
The milestone is tinged with sadness for the Queen as the calculation of the length of her reign is linked to the death of her father, King George VI, and that of Queen Victoria.
There was no mention of Victoria in the Queen's address but she was wearing her great-great grandmother's diamond bow brooch, which was handed down to her through the generations.
The Queen began her speech by remarking that she and Philip were delighted to be back in the Borders and to have arrived by train.
"It's been wonderful to witness the excitement which the return of the railway has brought here," she said.
Earlier she was greeted by delighted crowds at Edinburgh's Waverley Station as she stepped onto the polished steam locomotive Union of South Africa with Philip.
The Queen, the Duke and Ms Sturgeon travelled in a ''Pegasus'' Pullman coach that seats 14 in the main lounge area and also has a bar - that was serving tea and coffee.
The trio were seated in a pristine private compartment at the end of the coach's lounge area with the Queen's entourage in nearby seats.
Tributes have been paid to the monarch with Prime Minister David Cameron declaring in the House of Commons: "The Queen is our Queen and we could not be more proud of her.
"She has served this country with an unerring grace, dignity and decency and long may she continue to do so.''
Ms Sturgeon in her own speech greeted the Queen with ''a simple but heartfelt thank you'' for her work.
She said: ''We are privileged ma'am that you have chosen to mark today's milestone here. All of us are delighted to be able to share some of this special day with you. For those watching from around the world let me say, on their behalf ma'am a simple but heartfelt thank you.''
The rest of the royals carried on with their scheduled diary of engagements, with the Queen's daughter-in-law the Duchess of Cornwall describing the milestone as "marvellous" on a tour of the studios of ITV show This Morning.
The Queen's son the Duke of York, in a pre-recorded interview with the BBC, spoke of her consistency and leadership.
''It's a milestone in UK terms but as far as her consistency and leadership - it's the normal run of the mill sort of date. It's just one day in her reign," he said.
''So yes it's an extraordinary achievement in some respects, but actually it's about the consistency and the leadership that she is showing and has shown throughout her reign that I think is probably the one thing that marks her up more than anything else.''
The exact moment the Queen passes Victoria's milestone is not known as there is no precise time for the death of her father 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 when she will have reigned for a few minutes longer than Victoria.
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Scotland's First Minister highlighted in her speech, before the Queen opened the new railway, how Queen Victoria was also resident in Balmoral when she became the longest serving monarch in 1896 passing George III.
Speaking later Ms Sturgeon added: "I think it's an historic occasion, I think the Queen is clearly, as we saw there, very modest herself about it.
"But she has given many, many long years of dedicated public service and the attendance here today was a sign that people want to show their gratitude to her for that.
"She was in tremendous form, I think she enjoyed the train journey, the most special bit of a very special journey was when the train went through Galashiels station and slowed down and she was able to see some people who had gathered there outside the window."
Asked if an independent Scotland would keep the monarchy she replied: "My view and this was a view expressed during the referendum campaign last year is that, yes, the Queen, who is Queen of Scots, would remain head of state in an independent Scotland."
