If, as predicted, Boris Johnson becomes Prime Minister this Wednesday, he will travel to Buckingham Palace to be greeted by the Queen for the formal handing over of power, about an hour after Theresa May visits the monarch for the last time in office.
The timing of the Conservative leadership contest has forced the Queen to bend her rules a little – because she was supposed leave London last week for Balmoral where she spends her summers.
It would not have made sense for the 93-year-old monarch to travel 500 miles to the Highlands and back again for the handover.
But there is another rule which the Queen may have to break this summer, related to the delicate issue of sleeping arrangements at Balmoral should Mr Johnson become the 14th Prime Minister during her reign.
Prime Ministers and their partners traditionally visit the Queen at her Highlands estate in late summer, typically in the first week of September before the Commons returns after recess. It is believed Mr Johnson and his partner Carrie Symonds will be no different.
Yet, according to royal protocol, unmarried couples who are guests of the Queen at any of her six official residences do not stay overnight.
This bizarrely Victorian rule was even applied to Prince Harry and Meghan Markle, as they were then, at Christmas 2017 when, despite being engaged and invited for Christmas lunch at Sandringham, they stayed overnight at the nearby home of the Duke and Duchess of Cambridge, Anmer Hall. By contrast, a year later, the newly married Duke and Duchess of Sussex were allowed to sleep in Sandringham at Christmas.
Unless Mr Johnson and Ms Symonds get married in the intervening time, it is difficult to see how this convention remains in place unless the Queen takes a softer line on her 14th Prime Minister’s marital status.
An alternative would be for the visitors from Downing Street to stay in a lodge on the vast estate, although that may be seen as an unnecessary snub, particularly because Balmoral has 52 royal and guest bedrooms.
If he wins this week, Mr Johnson and Ms Symonds would be the first unmarried prime ministerial couple in Downing Street in modern times.
The pair first met when Ms Symonds was seconded to work on Mr Johnson’s Mayor of London re-election campaign in 2012.
It is understood Ms Symonds will not play a prominent role in this week’s events, preferring to take a step back and let Mr Johnson have his moment of victory – if all the predictions are right.
Buckingham Palace declined to comment on issues of protocol. However, as cohabiting couples are increasingly the norm in 21st-century Britain, it may be that the monarch has to change with the times.