Jean-Marc Ayrault, the French foreign secretary, broke with his usually mild-mannered tones to deliver strong words about Theresa May's appointment of the former Mayor of London to one of the most senior jobs in government.
He said Mr Johnson had lied to the British public ‒ with a key claim by Vote Leave to "give our NHS the £350 million the EU takes every week" proven wrong ‒ and warned that the French government needed a "credible" partner across the Channel.
"I am not at all worried about Boris Johnson," Mr Ayrault told Europe 1 radio, before appearing to change tack and express some apprehension at the appointment.
"But during the campaign he lied a lot to the British people and now it is he who has his back against the wall."
Mr Ayrault said pressure on Mr Johnson to defend the UK's interests should not worsen the relationship with the rest of Europe. Most EU leaders are frustrated both at the UK's decision to Leave and its government's apparent reluctance to initiate an official exit.
"He has his back against the wall to defend his country but also with his back against the wall, the relationship with Europe should be clear," said Mr Ayrault.
"I need a partner with whom I can negotiate and who is clear, credible and reliable.
"We cannot let this ambiguous, blurred situation drag on... in the interests of the British themselves."
Mr Johnson, who was a journalist before entering politics, was sacked in 1988 from his role as a correspondent for The Times after fabricating a quote.
He was also accused of closing all of London's ticket offices when he had promised they would be manned, seeing transport fares increase when he said they would reduce and rough sleeping double when he had promised to solve it, during his tenure as London Mayor.
Following the Brexit vote, Mr Johnson also wrote that immigration would not necessarily fall ‒ despite this being a key criticism of the Leave campaign against membership of the EU.
Mr Johnson's previous assertion that he would vote "in favour of the [EU] single market" does not square with Europe's politicians, including German Chancellor Angela Merkel, promising that free trade will not be granted without freedom of movement.
But the former Mayor of London is not set to negotiate the terms of Britain's exit from the EU under new Prime Minister Theresa May; that responsibility has been handed to fellow Leave campaigner and right-winger David Davis, who is heading a special department set up precisely for that purpose.
France continues to be one of the most vocal critics of Britain's wrangling over its place in the EU. Following the Leave result, Emmanuel Macron, the French economy minister, said Europe had allowed itself to be held to ransom by one, hostile, member state.
He said: "If we have failed, it was to have allowed a member to take the European project hostage in a unilateral way."
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