The appointment of Boris Johnson to the post of UK foreign secretary is no doubt a fine example of British humour: the former London mayor is to diplomacy what Stalin was to democracy.

It’s not every day that a country appoints as its global representative a known liar, a character for whom gross exaggeration, insult and racist innuendo seem utterly untroubling, a man apparently devoid of deep conviction about anything other than his own importance. “It wouldn’t surprise me now if Britain put Dracula in charge of the ministry of health,” scoffed the German politician Rolf Mützenich, spokesman for diplomatic affairs at the SPD.

But this British humour has a price: it devalues what Britain says in the international arena perhaps even more brutally than Brexit has devalued sterling. The reaction of the French foreign minister, Jean-Marc Ayrault, summed up the feeling of other EU governments: “You know his style,” he said, adding that Johnson had “lied a lot”, and that the EU would need a negotiating partner who was “clear, credible and reliable”. None of the evidence suggests that Johnson meets those criteria.

For many in EU circles the former London mayor’s reputation for mendacity pre-dates the referendum campaign. Nobody has forgotten his activities as a journalist in Brussels, where he was correspondent for the Daily Telegraph between 1989 and 1994. The French tend to mythologise “Anglo-Saxon” journalism as the pinnacle of ethics and rigour, but Johnson was the incarnation of the gutter-press dictum: never let the facts get in the way of a good story. Indeed, this is what a grinning Johnson often remarked to his foreign counterparts when they protested about his exaggerated stories.

I observed his methods when I was posted as a journalist to Brussels in 1992. One day he wrote a story claiming that Jacques Delors’ spokesman was so well-paid (as all of these incompetent Eurocrats, of course, had to be in Johnson’s narrative), that he lived in an immense chateau on the outskirts of Brussels. This was vehemently denied at a press briefing, to the hilarity of Johnson.

The story arguably had a grain of truth: Bruno Dethomas, the spokesman, lived in a large 19th-century house that had a turret on the outside, an architectural folly typical of the period in which it was built. “You see, it’s a castle!” Johnson laughed when I challenged him on the accuracy of his reporting.

Johnson can scarcely have believed what he himself wrote, but he kept churning it out. It was a game, a big laugh, especially as his fiercely anti-European newspaper lapped up these stories and gasped for more.

Fundamentally, despite our many conversations, I never actually knew what this son of a Eurocrat actually thought of the European project: was he as Europhobic as his own journalism suggested? Or was he just an opportunist trying to make a name for himself? Was he settling some old score with his father, an intelligent and much-respected EU official?

Either way, it wasn’t truthful journalism, but who cared about that? Johnson managed to invent an entire newspaper genre: the Euromyth, a story that had a tiny element of truth at the outset but which was magnified so far beyond reality that by the time it reached the reader it was false.

He had grasped the fact that some of his compatriots had a taste for conspiracy theories, and he could provide them – with a political scapegoat that was incapable of defending itself. The EU, unlike its member states, lacks an unquestionable political authority, so any reaction from the Brussels commission could immediately be dismissed as “interference” in the internal affairs of the nation state. Attempts to rebut the myths could be easily batted away as a case of no smoke without fire.

Even worse, Johnson created a school of EU reporting: the entire British press, to varying degrees, began peddling Euromyths, fuelling the kind of Europhobia that no UK politician dared to stand up to, and which ultimately has now led to Brexit.

Johnson as politician applied similar methods: so despite a previous ambiguity about the wisdom of Britain leaving the EU, he became head of the leave campaign, never hesitating to lie or insult Britain’s EU partners if it furthered the cause. And, just as when he was a journalist, he had no problem admitting his own lies the day after the referendum victory.

Yet none of it has prevented him ending up at the head of the British diplomatic service. The global face of Britain is now a buffoon (as many in Brussels describe him), whose word is as reliable as a used-car salesman’s.

But fair play to Theresa May: at least now we know that the Brexit negotiations will be brutal and dirty, and that the low blows will come thick and fast. What a delightful prospect.