The former head of the diplomatic service has said Boris Johnson was the worst prime minister he worked under.

Sir Simon McDonald served under seven prime ministers, from Margaret Thatcher to Mr Johnson, as a diplomat and, from 2015 to 2021, as permanent under-secretary to the Foreign Office and head of the diplomatic service.

A civil servant for nearly four decades, Sir Simon has had a unique insight into the workings of government and after resigning last year is now a cross-bench peer in the House of Lords.

He spoke to the Beth Rigby Interviews… programme about the PMs he worked with and the importance of staying neutral as a civil servant.

Sir Simon, whose book Leadership: Lessons from a Life in Diplomacy is published next week, worked closely with Mrs Thatcher, John Major, Tony Blair, Gordon Brown, David Cameron, Theresa May and Mr Johnson.

“Thatcher was the best and Johnson was the worst,” he said.

And if he had to include Liz Truss, then “she was a worse prime minister than Boris Johnson” while Rishi Sunak is “methodical and promising”.

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Johnson was ‘charismatic but chaotic’

Sir Simon, who is now the master of Christ’s College, Cambridge, said Mr Johnson, on a personal level, was “always charming to deal with, he was humorous, he was kind, he was the foreign secretary I worked with who had the most time for the people in his office”.

“And this is a real mark of a character. But what you need to be an effective prime minister is different. Being prime minister is one of the toughest jobs in the world,” he said.

“He is charismatic but chaotic.

“He liked to have multiple opinions swirling around him, the people proposing those ideas never really knew whose 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.”

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The downfall of Boris Johnson

In July this year, Sir Simon took the unusual decision, for a former civil servant, to tweet out a letter to the standards commissioner saying Number 10 “are still not telling the truth” about Mr Johnson not knowing about previous sexual assault allegations against Conservative MP Chris Pincher.

Asked if Mr Johnson was told about the allegations against Mr Pincher, the subsequent investigation and the outcome, Sir Simon said he was informed when he was foreign secretary and again when he was prime minister.

Sir Simon’s tweet put Mr Johnson’s premiership in peril, with the row over Mr Pincher leading to the former PM’s exit from office.

He said he did not think his letter would have such an impact and admitted the backlash from the government was “unpleasant” but not as bad as what the victims of Mr Pincher had experienced.

“I spoke for a couple of reasons. First of all, I’d left the civil service and am now a member of the House of Lords. I am part of the legislature, so I have additional duties,” he said.

“Second, as the story developed, it seemed that nobody was paying attention to the previous victims. And there were victims.

“And I thought they should not be airbrushed, but what they had endured should be remembered.

“I’d written the letter on Monday evening, my wife made me sleep on it and as we were going to sleep I said ‘do you think anybody will notice?'”

Mr Pincher, a former deputy chief whip, denies all allegations of sexual misconduct.

‘My letter was the final straw’

A former minister then told a newspaper Sir Simon and Mr Johnson “never saw eye to eye” as the civil servant was a Remainer, implying he had an ulterior motive.

“It was unpleasant but much less unpleasant than what the victims of the various Pincher scandals had undergone. And it was wrong. I knew it was wrong,” Sir Simon said.

He added that Mr Johnson knew he was a Remainer but denied pushing those views as he strongly believed his job was “to make the best of the exit, that is what civil servants do, no matter the government, even when they disagree”.

Despite initially questioning whether his letter would have any impact, Sir Simon added: “I accept that mine was the final straw that made it onto the Johnson camel’s back first.”

When Sir Simon stepped down as head of the diplomatic service early, there was speculation it was because he was against the merger of the Foreign Office with the Department for International Development.

But he denied that was the case, saying: “I supported the merger very, very strongly.”

Thatcher to Johnson

Sir Simon said all the PMs he has served under were “good at some things and weak at other things” – and “nearly all look better in the rear view mirror”.

“Margaret Thatcher was a very difficult prickly character for the system, but who had a clarity and a sense of purpose and a sense of galvanising the system which looks to have been one of our most effective prime ministers in 300 years,” he said.

He said John Major looked “beleaguered at the time” but “was one of the most methodical men I’ve ever seen”.

Tony Blair was “the best at communicating but some of his biggest policy calls were just wrong,” he said.

He said he does not think Mr Blair lied when he said there were weapons of mass destruction in Iraq but “the intelligence picture comprehensively misled him” and he “believed what he was saying”.

“It’s very, very difficult as a human being to admit wrong decisions,” Sir Simon added about Mr Blair not admitting he was wrong.

He said Gordon Brown was “the best of finance but quite a difficult communicator” who “wore his anxieties on his face” which was not helpful when leading people “through very difficult time”.

David Cameron, Sir Simon said, “looked the most of ease in the job and was in some ways the easiest to work for”.

Theresa May was also a “methodical person but with a very difficult job that she didn’t really sympathise with”.