The Queen will receive the new prime minister at her Balmoral estate in Scotland for the first time in her long reign.
Her Majesty will meet either Liz Truss or Rishi Sunak on 6 September, a day after one of them is announced as the winner in the race for 10 Downing Street.
She would normally appoint prime ministers from Buckingham Palace or Windsor Castle.
However, the 96-year-old monarch has been advised to remain at her Balmoral residence in Aberdeenshire, where she is enjoying her summer holiday.
Whoever wins, they will become the 15th British prime minister appointed by the Queen, who first confirmed Sir Winston Churchill into office in 1951.
It will be from there that she will install either Rishi Sunak or Liz Truss as prime minister on 6 September, breaking years of royal precedent.
On Wednesday, a Buckingham Palace spokesperson said: “The Queen will receive Prime Minister Boris Johnson on Tuesday September 6 at Balmoral, followed by an audience with the new PM.”
It is understood the decision was taken early to provide certainty for the prime minister’s diary.
If the Queen had experienced a mobility issue next week, and London or Windsor had been chosen, then alternative arrangements would have been needed, possibly at the last minute.
Regarding Her Majesty’s health, Sky’s Scotland Correspondent, James Matthews, tweeted: “The Queen has been seen at Balmoral recently looking “remarkably well” according to observers, albeit with reduced mobility.
“Using stick on rough ground, but without whilst walking the corgies. Picnicking with lady in waiting at her mother’s fishing lodge.”
Since last autumn, the Queen has suffered with mobility issues and now regularly uses a walking stick.
This year, she has been forced to cut several official engagements short, and notably missed much of her Platinum Jubilee celebrations, only appearing briefly on the palace balcony.
In June, she missed Royal Ascot for the first time since her coronation, with the Duke of Kent taking her place in leading the royal carriage procession.
She also contracted COVID-19 back in February, and later spoke about how it left her feeling “very tired and exhausted”.
The winner of the Tory leadership contest to succeed Boris Johnson is set to be announced on Monday 5 September.
His successor will then travel to Balmoral to be officially appointed by the Queen the following day.
