He met Queen Elizabeth II that day, as is traditional.
In the UK the Queen must currently invite an incumbent Prime Minister to form a government before he does so.
And Boris and Her Majesty will likely be seeing each other again soon if precedent is anything to go by.
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Generally in the summer British Prime Ministers are invited to Balmoral, the Queen’s huge private country home in Aberdeenshire, Scotland.
However, there has been much speculation as to whether Boris Johnson’s girlfriend, who he is 24 years older than, will be invited. The Queen has refused to host unmarried couples in the past, including even her grandson Prince Harry and Meghan Markle.
Carrie’s chances at an invitation may well be further compromised by the fact that not only are she and Boris Johnson unmarried, the Prime Minister is actually still married to his second wife.
As a stickler for tradition, as well as the head of the Church of England, it is thought the Queen is unlikely to look favourably on this domestic set up.
Boris Johnson and Carrie have bought a £1.3 million house together in Camberwell, London, although it is thought she lives in Number 10 with him.
However, many are placing bets on Carrie being extended an invite despite her modern relationship status with the PM.
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Royal residences: Balmoral is a private property of the Queen's in Aberdeenshire, Scotland
Boris Johnson: Queen refused to host an unmarried Harry and Meghan at Balmoral
Be My Bet said: “One of the most popular bets this week is whether Carrie Symonds will stay at Balmoral and break royal protocol, odds have shortened on this.”
It is currently at 1/2 one that Carrie will be invited to Balmoral, and 6/4 that she is not.
A BeMyBet.com spokesperson said: “The odds suggest the Queen could well be moving with the times by breaking royal protocol and letting unmarried Carrie stay overnight in Balmoral.
“The ban applied even to her own grandson Harry and Meghan during their Christmas 2017 visit, but Her Majesty has been tipped to bend the rules for Boris in what would be an unprecedented move.”
The public have been offering up their opinions on this unusual situation.