Speaking in an interview with The Sunday Times, the former US presidential candidate called Mr Johnson and the President “blowhards” and decried their “authoritarian” politics.
Describing her initial impressions of the Prime Minister when she first encountered him during his time as mayor of London, Mrs Clinton said: “I thought he was a grandstander and very full of himself.
“I knew Trump [before he became president] and I thought the same thing.
“A kind of blowhard, a blustering guy. But they both clearly harboured great ambitions for themselves.”
Mrs Clinton said both Mr Trump and Mr Johnson had developed strategies “straight from the playbook of wannabe authoritarians” and that both had shown “a total disregard for facts”.
The Sunday Times’ Decca Aitkenhead told Mrs Clinton that Mr Johnson had been portrayed as the “people’s hero in a battle of the people versus parliament” during the Conservative leadership campaign.
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“Well, then I fear for your country,” Mrs Clinton replied.
“If anyone says he’s a hero to anyone other than himself, I really worry. We don’t know what’s going to happen with Brexit, or impeachment, or our upcoming election.
“And I don’t care what your political ideology is. If you invest your support in people who will try to take advantage of it to promote themselves to become authoritarian leaders, then we are headed for an even worse outcome.”
It comes as British and EU officials continued Brexit talks over the weekend amid rising speculation a deal is on the cards that could break the deadlock over the Irish border.
The EU’s chief negotiator Michel Barnier gave the green light on Friday for the start of intensive discussions between officials.
It followed an apparent breakthrough in talks on Thursday between Boris Johnson and his Irish counterpart Leo Varadkar at a country house retreat on the Wirral.
Their discussions appeared to unblock the negotiations, which had seemed to have been running into the ground following the publication of Boris Johnson’s Brexit blueprint.
However, the Prime Minister cautioned it was not a “done deal” and said there was still “a way to go” if they were to get an agreement that would enable Britain to leave on 31 October, as he has promised.