It was not just the persistently overcast skies – a weather pattern once dubbed the “Brabant gloom” by Roy Jenkins, a former European commission president – that made working in Brussels for the Daily Telegraph in the early 1990s a joyless experience. It was my role as deputy to Boris Johnson, then “bureau chief” in name but solo performer in practice, that ensured my first job as a foreign correspondent was a trial of endurance.

There were just the two of us in the Telegraph office, and we were working long hard hours reporting on the political and economic convulsions of the Maastricht treaty negotiations. The story itself, of negotiations that played out in meeting rooms of Brussels, was full of political intrigue and drama. And whatever happened was likely to shape Europe for years to come.

How Johnson wrote about it, though, not only alarmed me at the time but helped set in stone a pervasive anti-European narrative that never really encountered serious challenge in the UK. For shamelessly painting the European commission as an insanely grandiose and imperialist body, he was rewarded by flurries of “herograms” from our editor, Max Hastings, including one that read “we all think you’re doing a wonderful job if only you’d learn to be a little more pompous”. Johnson’s trajectory to the gates of Downing Street had begun.

Over the months and years, those inventive stories, of fishermen forced to wear hairnets or snails reclassified as fish, created a deeply rooted belief that anything out of Brussels must be either loony or the result of a sinister continental plot. His most explosive story of all, published in May 1992, claimed that Jacques Delors, then commission president, was conspiring to centralise huge powers in Brussels, in effect creating a European superstate.

Even Johnson’s supporters admitted that the story was distorted. He blamed his editors for the wording and never even tried to defend it when challenged. But it was repeated as fact and won him the adulation of the far right across Europe. In Britain it fanned Little Englander flames and many draw direct lines from this casting of Brussels as an invading force to the current perilous moment in British history in which “freedom” from Europe at any price is apparently desired by millions, even at the expense of the economy and the constitutional unravelling of the United Kingdom.

It is a circular irony – and perhaps even an inevitability – that Johnson, now 55, looks set to be the prime minister entrusted with resolving the national crisis he spawned with his writing two decades ago.

I first met him in 1992, when he was 28, and it was clear to me very quickly that he had ambitions beyond plying his trade as a hack – there was always an agenda, a talent for self-promotion and an obsession with power that marked him out. Most journalists’ preference is to observe rather than to take part.

By 2002 I was inviting readers of another newspaper to imagine him on the steps of Downing Street with his then-wife Marina and four well-scrubbed kids. Such a prediction was ridiculed as preposterous by the commentariat of the time, who – never having seen him at close quarters – were largely taken in by Johnson’s carefully curated facade of intelligent bumbling. I tried to explain to largely deaf ears that “beneath that well-cultivated veneer of disorganisation and dysfunction lies not so much a streak of ruthlessness as a torrent of frighteningly focused ambition”.

Johnson has always wanted to win at all costs, by fooling people into believing his performance or by sabotaging rivals if necessary. Indeed, the first time I met him, me fresh off the plane as an eager new correspondent, he knowingly sent me in the wrong direction for a press conference in one of the main EU buildings so that I arrived unprofessionally late.

There were already clues that it was only the winning, the exercising of supremacy over others, that really mattered. A lack of real conviction about Europe or most other matters seemed to translate into a lack of ideas beyond his own self-projection – what was the point of winning but the winning itself?

Johnson never seemed really to believe what he was writing or saying. His motivation seemed solely to boost his own fame and fortune, having identified a lucrative “gap in the market”. Most other journalists were writing boringly about “the European project”, which left plenty of scope for glory by deploying wit to rabble-rouse. The language of Johnson’s copy was full of “plots” and “traps” laid by the French against the “limp-wristed” English with their “shy grins” and “corrugated soled shoes”. And so he succeeded in making the sceptic cause an attractive and emotionally resonant one for the right by pandering to – or perhaps even creating – the twin national phenomena of arrogance combined with strange feelings of inferiority.

Yet his journalism – soon branded the “English position” by prescient Eurocrats – actually bore little relation to the affection he expressed for the EU in private moments in the office while I made coffee and raided the stash of Côte d’Or chocolate in the cupboard. The tension between these two contradictory positions was, evidently, uncomfortable for him, even if he found the mayhem he caused not just entertaining but exhilarating. The sound of smashing glass after throwing rocks into next door’s greenhouse gave him, he once confessed, a “weird sense of power”.

There are large elements of the disruptor in the Johnson of 2019 – a sense that he is almost alone in enjoying the chaos caused by the Brexit made by Boris. That pleasure only seems to fade when he is presented with the realities of the difficulties he will face as prime minister in restoring calm and prosperity to an exhausted and fractious country. He is the teenager who wants the glory of hosting the wildest party but avoids facing his parents’ wrath or paying the bills for the repairs.

It is also disturbing that similar patterns of personal behaviour to those I witnessed as his colleague during those febrile times in Brussels are apparently resurfacing now that he feels the pressure of cold scrutiny for the first time. These character traits – irrational outbursts, aggressive behaviour, flashpoints of instability – must lead to questions about his suitability for the highest office in the land. Strange, often small things, would trigger extraordinary episodes of loss of self-control – often simply that some aspect of his super-secretive life had come out into the open or that some whim had been denied. He did not like anyone knowing what he was up to, where he was, or even whom he was talking to. He locked the door to his room in the office, for instance, and also kept the office answerphone under lock and key so I could not listen to messages.

In just three weeks this frankly bizarre and troubled man looks set to hold the keys to No 10 and have access to the nuclear codes. He might have in large part created this mess but he is certainly not qualified to fix it.

• Sonia Purnell is the author of Just Boris: A Tale of Blond Ambition