THE opening of the Borders Railway by the Queen will, by happy coincidence, be on the day that her Majesty becomes the longest-reigning British monarch.

On September 9, Queen Elizabeth will surpass the record of her great-great-grandmother Queen Victoria who reigned for 23,226 days, 16 hours and 23 minutes. The problem for those organising her “record” day is that no one knows precisely the second when the duration of Victoria’s reign will be overtaken.

Though she will certainly be in Scotland, the exact moment that the record is set will be unknown, which has forced Buckingham Palace into a ‘guesstimate’ that will become the official record time.

According to long-held tradition and what passes for the British constitution, a monarch ascends the throne at the instant his or her predecessor dies, hence the cry: “The Queen (or King) is dead, long live the King (or Queen).”

In the case of the present Queen’s father, King George VI, the exact time of his death is unknown, because although he was suffering from lung cancer, he died peacefully in his sleep from a coronary thrombosis. He was last seen alive around midnight before his body was discovered in his bed by a servant bringing him a cup of tea at 7.30am on the morning of February 6, 1952.

Most historians date the Queen’s rule from that time of the day, but given the interest in the new record reign, The National can reveal that Buckingham Palace has decided to calculate an approximate time when the record will be beaten and the time agreed is 5.30pm on September 9.

Royal Household experts passed this written statement to The National: “Her Majesty came to the throne on the death of her father, King George VI, on 6 February 1952.

“The exact time of his death is not known: he was seen at his bedroom window around midnight on 5 February, and is thought to have died in the very early hours of 6 February in his sleep (so exact minutes become somewhat academic in the calculation).

“Assuming the time was around 1.00 am that morning, The Queen will (God willing) pass her great, great grandmother’s record around 5.30 in the evening of 9 September 2015.”

The Royal Household has gone to considerable lengths with its calculations – the records of King George VI’s death show that doctors were able to say that he had died some hours before the discovery of his body, and that is why the 1am has been chosen.

They have also calculated every day and minute of both the current Queen and Queen Victoria’s reigns.

Buckingham Palace had let it be known that Her Majesty had not wanted a celebration of the record as such and had decreed a business-as-usual day during her stay in Scotland.

The exact whereabouts of the Queen at the moment she becomes Britain’s longest-reigning monarch have not yet been disclosed, but all those involved with the Borders Railway is hoping for a massive boost from the international publicity that will attend the record-setting day.

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.”