The Prime Minister made the revelation in an election campaign video filmed on his way to meet the head of state on Wednesday, saying “she always asks the best questions.”
It signalled the start of the General Election as Mr Johnson launched the Conservative Party's official campaign.
It is not the first time the PM has appeared to break the rules, after revealing details of his conversation with the Queen when he was elected in July.
In the footage posted to his own Twitter account, the Tory leader can be seen speaking from the back seat of his car as he heads to Buckingham Palace.
In the video, Mr Johnson says: “I’m just off to see Her Majesty the Queen, which is always a very tough interview because she always asks the best questions and the question today is: why are we having this election?”
Buckingham Palace declined to comment on Mr Johnson’s supposed breach of protocol.
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Conversations between the Prime Minister and the monarch are supposed to remain private as she is supposed to remain above party politics - with Mr Johnson breaking protocol following his first official meeting as head of state.
In July, when Mr Johnson accepted the Queen’s invitation to form the next government and become Prime Minister, he commented about their conversation when he returned to Downing Street.
Exclusive: “I don’t know why anyone would want the job” - what Johnson says HM the Queen told him during his audience! The PM revealed it during a tour in Number 10 before being told off by staff not to repeat those things so loudly. 📷:PA pic.twitter.com/r5CX4LbnXa— Vincent McAviney (@VinnyMcAv) July 24, 2019
At the time, a tweet by Vincent McAviney, a UK correspondent for Euronews NBC, made the claim: “‘I don’t know why anyone would want the job’ – what Johnson says HM the Queen told him during his audience!
"The PM revealed it during a tour in Number 10 before being told off by staff not to repeat those things so loudly.”
Ex-Prime Minister and Tory leader David Cameron had to apologise to the Queen after he revealed to former New York Mayor Michael Bloomberg that she had “purred down the line” when he informed the monarch Scotland had voted against independence.