The former foreign secretary told the Sunday Times that he took a call from someone claiming to be the Downing Street switchboard.

“The person had a mild Irish accent and said, ‘This is the No 10 switchboard, we have got the president of the United States for you. I am just going to patch you through to the White House situation room,’” Johnson said.

“Thinking quickly as I do, I said, ‘To all our listeners on Radio Kilkenny, I was not fooled for an instant.’”

Johnson said he then hung up and called No 10, to be told that “it was the president.”