The Queen will miss the popular Highland Games Braemar Gathering in Scotland on Saturday which the Prince of Wales was also scheduled to attend.
Sky News understands the decision has been taken for the comfort of Her Majesty, who has been suffering from mobility problems since last year.
In what has been described as a “huge break with tradition”, the Queen announced she would appoint the new prime minister at Balmoral.
The monarch will receive either Liz Truss or Rishi Sunak on Tuesday, 6 September – a day after one them is announced as Boris Johnson’s replacement and handed the keys to 10 Downing Street.
New prime ministers are usually appointed from Buckingham Palace or Windsor Castle.
However, the monarch, who celebrated her 96th birthday in April, has been advised to stay at Balmoral, in Aberdeenshire.
Whoever wins the race for the Conservative leadership will officially become the 15th British prime minister appointed by the Queen, who first confirmed Winston Churchill into office in 1951.
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Her Majesty this year celebrated her Platinum Jubilee but pulled out of a number of events to mark her historic 70 years on the throne.
The guest of honour “reluctantly” abandoned plans to attend a thanksgiving service at St Paul’s Cathedral after experiencing discomfort during the first day of Jubilee events, including a birthday parade and flypast.
The monarch was also forced to miss Royal Ascot for the first time since her coronation in June.
And she withdrew from the traditional Easter Sunday service at St George’s Chapel in Windsor this year.
Instead, grandson Prince William and wife Kate led the royals at the annual royal event.
