The Queen overtook her great-great grandmother Queen Victoria today as she became the UK’s longest serving monarch.

David Cameron will lead the world’s tributes to a monarch who has served the Commonwealth through the Cold War, the Space Age and the Internet Age

In her 21st birthday broadcast to the Commonwealth, Princess Elizabeth memorably promised that “my whole life, whether it be long or short, shall be devoted to your service”.

She could not have expected as she said those words that the burden of monarchy would fall on her shoulders so soon afterwards, but more than 63 years after she ascended the throne, the Queen has never wavered from that pledge.

On a day of history, of celebration and gratitude as she becomes our longest-reigning monarch, David Cameron will express the thanks of a nation by describing the Queen as “a rock of stability in a world of constant change”.

In turn, Her Majesty is expected to make a brief reference to her achievement in surpassing her great-great grandmother Queen Victoria when she opens a new railway in the Scottish Borders.

She will reflect on what she acknowledges as “a remarkable life”, though for her the occasion will be tinged with sadness because it is “a record she would rather not have had”. Her record-breaking reign was only possible because her father George VI died at the age of just 56.

Meanwhile Buckingham Palace released a new portrait of the Queen to mark the occasion, taken by Mary McCartney, who described the monarch as “a trailblazer and a beacon” for women.

The Prime Minister, leading tributes to the Queen from the House of Commons, will say: “Millions of people across Britain will today mark the historic moment when Queen Elizabeth II becomes our longest serving monarch.

“Over the last 63 years, Her Majesty has been a rock of stability in a world of constant change and her selfless sense of service and duty has earned admiration not only in Britain, but right across the globe.

“It is only right that today we should celebrate her extraordinary record, as well as the grace and dignity with which she serves our country.”

The Queen had wanted to spend the day privately at Balmoral, as she does not regard outliving her ancestor as a day for celebration, but has bowed to public demand by agreeing to open the Scottish Borders Railway, on which she will travel from Edinburgh to Tweedbank, near Galashiels

A royal source said the sovereign would have mixed feelings as she took in the cheers of the crowds.

The source said: “As she sits on the train I imagine she will be thinking ‘Isn’t this beautiful countryside, isn’t it remarkable that these people turn out in these numbers to mark an occasion like this, and what a remarkable life.”

But the celebratory mood will be mixed with “a bit of melancholy”, the source said.

February 6, 1952, the day her record-breaking reign began, remains for her a mournful anniversary, as it was the day her father George VI died. The Queen, as she had suddenly become, was aged just 25 and was on a trip to Kenya at the time.

She immediately flew back to London, where she was photographed looking strained as she arrived at Clarence House the next day, and formally proclaimed herself Queen and Head of the Commonwealth and Defender of the Faith at 10am on February 8 during a meeting of the Lords of the Council at St James’s Palace. Mourning her father had to be put on hold.

The royal source said: “This is a record that only exists because the Queen’s father died young. Given the choice, she would far rather her father had lived a long life and she did not have this record, so it’s a record she would rather not have had.

“It is also a record that is determined by the death of her great-great grandmother, and that is something she has always been conscious of.”

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