David Cameron has described Queen Elizabeth as a "rock of stability" and praised her "extraordinary record" as the British monarch becomes the longest serving head of state since Queen Victoria. The prime minister paid tribute to the 89-year-old ahead of a discussion on her reign in the House of Commons on 9 September.

"Millions of people across Britain will today mark the historic moment when Queen Elizabeth becomes our longest serving monarch," the Conservative leader said.

"Her Majesty has been a rock of stability in a world of constant change, earning admiration for her selfless sense of service and duty. It is only right that we should celebrate her extraordinary record, as well as the grace and dignity with which she serves our country."

The Queen has reigned for exactly 63 years and 217 days after her King George VI passed away on 9 September, 1953. But despite the impressive milestone, Buckingham Palace have said the monarch will continue with her engagements and open the new Scottish Borders Railway.

The First Minister of Scotland and SNP leader Nicola Sturgeon said she was looking forward to officially opening the railway with Her Majesty on what the nationalist described as a "very special day".

But Graham Smith, the chief executive of campaign group Republic, has told IBTimes UK: "Being on the throne the longest is not an achievement. It just comes down to good timing, good luck and genetics. If you're given a job for life at the age of 25 and you're still around at the age of 85 or 90 then, that is what happens."

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