The ex-top official at the Foreign Office who served under seven Prime Ministers has described Boris Johnson as the "worst" leader he worked for.
Sir Simon McDonald, whose explosive intervention helped to bring down Mr Johnson in the summer, said the former PM was "charismatic" but "chaotic".
As a civil servant for nearly four decades, he served under Margaret Thatcher, John Major, Tony Blair, Gordon Brown, David Cameron, Theresa May and Mr Johnson.
Speaking to Sky News Beth Rigby Interviews, the former head of the diplomatic service said all were "good at some things and weak at others".
Sir Simon, who became a diplomat in 1982 and worked as permanent secretary at the Foreign Office until 2020, added: "Thatcher was the best and Johnson was the worst".
But if he had to include Liz Truss, who he did not work under, then she "was a worse Prime Minister than Boris Johnson" - after serving just 49 days in No10.
He said the new Prime Minister Rishi Sunak is "methodical and promising".
Sir Simon, who worked alongside Mr Johnson during his stint as Foreign Secretary in Theresa May's Government, said he was "always charming to deal with" and "kind".
But he added: "What you need to be an effective Prime Minister is different. Being Prime Minister is one of the toughest jobs in the world.
"He liked to have multiple opinions swirling around him, the people proposing those ideas never really knew who was in the lead - sometimes the decision wasn't clear and sometimes the decision was reversed.
"There was too much swirl, and in the end, the system responds to clearer directions.
"One of the most disconcerting things was to see him arrive at a meeting, pretending to be less well briefed than he actually was. But that was part of his character.
Days before Mr Johnson's resignation, Sir Simon published a bombshell letter claiming No10 was "not telling the truth" about the then-Prime Minister's knowledge of allegations of inappropriate behaviour by the chief whip Christopher Pincher.
Mr Pincher, who denied the claims, resigned from the government, but the scandal ultimately led to the resignations of Rishi Sunak and Sajid Javid from Cabinet.
Sir Simon told Sky News: "I accept that mine was the final straw that made into onto the Johnson camel's back first".
