ALTHOUGH the exact moment when Queen Elizabeth today becomes the longest-reigning monarch in British history is undetermined, the official moment of her replacing Queen Victoria in the record books will be 5.30pm this evening.

Her exact location at that time is a secret as Buckingham Place has confirmed that the Queen, 89, will be back on the private section of her time in Scotland.

There is no doubt, however, that the Queen will definitely be in Scotland and informed speculation is that she will either be en route north or already back at Balmoral Castle after spending most of the day opening the new Borders Railway.

The National revealed last month that the 5.30pm time had been chosen by expert court staff because no-one can ever know the exact moment that Queen Elizabeth replaced her father King George VI as monarch – he died peacefully in his sleep in the early hours of February 6, 1952.

As he was last seen alive shortly after midnight and was not found dead until 7.30am, his doctors were forced to calculate the time of death from the coldness of his body and deduced that he had died around 1.30am.

As Queen Victoria’s reign lasted 23,226 days, 16 hours and 23 minutes – her ascent to the throne and her death were meticulously recorded – the record is therefore set to be broken around 5.30pm local time. The lack of fuss over the exact record-breaking time is a deliberate policy of the Queen herself. She emphasised many months ago that there should be no celebrations and that her tasks would go on as normal, apparently out of respect for Queen Victoria.

The result is a likely tourism bonanza for the Borders, with camera crews and print media coming from all around the globe to mark a moment in history.

There is also bound to be some political gnashing of teeth down Westminster way at the fact that First Minister Nicola Sturgeon will be accompanying the Queen most of the day.

The Queen and Prince Philip will arrive at Waverley Station in Edinburgh later this morning to board the train drawn by the steam locomotive Union of South Africa. The royal party will then travel down the new £300 million route, stopping off at Newtongrange in Midlothian to unveil a plaque at the station.

The train will then carry on to the final stop on the new railway at Tweedbank for the official opening scheduled for just after 12.30pm. The Queen is expected to stay at least an hour and then resume her holiday – by convention, details of where and when she spends that holiday are not given to the press, except perhaps when a referendum is looming, but Balmoral Castle would appear to be where the Queen will be staying tonight.

It is unlikely that the longevity record will fail to be mentioned by the various dignitaries, and there was speculation that the Queen herself would make reference to it.

Councillor Graham Garvie, convener of Scottish Borders Council, said: “The Borders Railway is the most significant project for generations in our region and we are thrilled to welcome Her Majesty The Queen to formally open the railway. The fact that this occasion coincides with Her Majesty becoming Britain’s longest-serving monarch makes it unique in the history of the Borders.”

Phil Verster, managing director of the ScotRail Alliance, said: “We are thrilled that Her Majesty The Queen will open the new Borders Railway on such a special day.

“The Borders Railway will open up Midlothian and the Borders to so many more opportunities, including tourism, leisure, work and education, and marking the opening of the line with a royal visit really is wonderful.

“This will be a truly historic day and we’re honoured to share it with Her Majesty.”