Queen Elizabeth II will receive the UK's next prime minister at Balmoral rather than Buckingham Palace -- a historic first for her 70-year reign.

A spokesperson for Buckingham Palace said the 96-year-old monarch will not be making the 1,000-mile round trip from Scotland and instead the outgoing leader Boris Johnson will travel north from London on September 6, followed by an audience with his successor.

The decision was taken for the audiences to take place at Balmoral in order to provide certainty for the Prime Minister's diary, a royal source told CNN.

The source added that if the monarch were to be experiencing an episodic mobility issue next week and the plan had been to travel to London or Windsor, it would have meant alternative arrangements being made at the last minute.

Johnson was left with little choice but to step down as Conservative Party leader in early July after months of scandals that rocked his government and led to dozens of ministerial resignations. His announcement triggered a leadership contest that saw a broad field of contenders whittled down to two: Foreign Secretary Liz Truss and former Chancellor Rishi Sunak.

The pair have spent the summer trying to curry favor with 160,000 rank-and-file members of the party. The result of that vote will be announced on Monday.

As the leader of the largest party in Parliament, the winner will be invited by the Queen to form the next government and become her 15th prime minister.

Appointing a new prime minister is one of the Queen's core ceremonial responsibilities as Head of State -- with others being the State Opening of Parliament and the signing of parliamentary bills into law. In May, Princes Charles and William took center stage at the grand set-piece event, with the heir to the throne reading the government's legislative agenda for the year ahead on his mother's behalf.

It was the first time the Queen had missed the event in 59 years. The palace cited the monarch's "episodic mobility problems" but declined to divulge further details citing patient confidentiality. These ongoing issues, which she has faced for nearly a year since a brief hospital stay last October, have also hampered other occasions including her Platinum Jubilee celebrations in June.

Queen Elizabeth II' s 14 prime ministers

The ceremony next week will be the first time a prime minister has officially tendered their resignation or been appointed outside of Buckingham Palace during the Queen's seven decades on the throne.

Several events over the last 12 months have been modified for the Queen's “comfort” with one of the most recent examples being her arrival at Balmoral Castle earlier this month. The moment is traditionally marked by an honor guard at the gates of the 50,000-acre estate but this year's inspection of the troops was held privately. (CNN)

•PHOTO: Britain's Queen Elizabeth II smiles at the official opening of the new building of Thames Hospice in Maidenhead, Berkshire, on July 15.

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