LONDON—Boris Johnson, the far-ahead front-runner to become Britain’s prime minister this week, waved a vacuum-packed fish over his head and railed at the European Union.

A kipper smoker on the Isle of Man, Johnson said Wednesday, “has had his costs massively increased by Brussels bureaucrats who have insisted that each herring fillet must be accompanied by” — he paused to reach for another prop — “a plastic ice pillow! Pointless, expensive, environmentally damaging, health and safety.”

The audience roared. Johnson, populist charmer, always gets a laugh.

Did it matter that the ice pillow was actually the result of a British regulation and had nothing to do with the EU? Does it matter that the likely future British leader has a loose relationship with the truth?

The key to understanding Johnson, say his biographers, lifelong observers, friends and enemies in a dozen interviews with the Washington Post, is to see him first as a hack — a hack being the self-deprecating but not pejorative Britishism for a working journalist shovelling reams of copy to his masters on deadline.

Johnson was fired from his first job, at the Times of London, for making up a quote about Edward II’s catamite lover and attributing it to his godfather, the Oxford historian Colin Lucas.

But he would go on to find his voice, and develop his shtick, during his years as a Brussels-based foreign correspondent — racing around in his lipstick-red Alfa Romeo, speaking intentionally bad French and banging out outrageous and only semi-true dispatches.

Before he was a lawmaker, London mayor or foreign secretary, Johnson made his name as one of Britain’s top columnists. And he has continued as a hack through much of his political career. A possible last column ran just a week ago.

It was as a hack, writing for the middlebrow “Tory Telegraph,” that Johnson learned to combine his high and his low. He is an upper-class Oxford-educated classicist who sprinkles his rapid-fire remarks with Latin aphorisms. But he has also cultivated a persona as a populist everyman in frayed trousers who bikes to the office. He is a version of his favourite meal: links of proper British sausage quaffed with $100-a-bottle Tignanello.

It was as a hack, too, that Johnson stoked the cheeky, slanted, self-pitying euroskepticism that would set the stage for Brexit — and ultimately send him in the direction of 10 Downing Street.

“Beware of newspaper columnists,” Martin Fletcher, a former Times editor, told the Post. “They are paid to be controversial, to be colourful, to be provocative. There are very few consequences to what they write. But those are not attributes you want in your prime minister.”

The ranks of the British political class are full of former journalists. But Johnson — if he bests Foreign Secretary Jeremy Hunt when the results of the leadership race are announced Tuesday — would be the first hack in the top job in recent times. In this, he is thought to aspire to be like Winston Churchill. Johnson, by all accounts, has never been lacking in ambition.

Hardly anyone had heard of him when 24-year-old Johnson arrived in Brussels in the spring of 1989 to begin life as a foreign correspondent for the Telegraph. He and his new first wife moved into an inauspicious flat above a dentist’s office in Woluwe-Saint-Pierre, a bourgeois neighbourhood where fussy Flemish residents would scribble anonymous notes complaining about the misplacement of trash bins, according to his biographers.

By the time he left Brussels five years later, Johnson was one of Margaret Thatcher’s favourite commentators — the banner man for a new type of Tory, an irreverent, kind of hip Conservative who would shortly launch his rocket of a political career.

In Brussels — the often dull, bureaucratic capital of the EU — the young Johnson found a way to run at the front of the field, ahead of older, more docile correspondents who tended to stick up for the EU and stick to the facts.

Johnson hit on a theme: that the European Union was run by the devilish French and rules-obsessed Germans, who were out to pass all sorts of onerous rules designed to clip the wings of a once-great Britain.

Skewering European officials at theatrical news conferences, then letting it rip on his Tandy 300, Johnson became the subject of envy and admiration in the British press corps in Brussels.

“His stories about the idiocies of the European Union were received with rapture by an ever-growing circle of fans, and he became the only Brussels correspondent of whom ordinary mortals have heard,” wrote Andrew Gimson, a former colleague and author of a biography, The Adventures of Boris Johnson.

But these same stories were viewed, by colleagues and competitors, as deeply dubious.

Peter Guilford, who worked alongside Johnson as a Brussels correspondent for the Times, credited Johnson with turning euroskeptic journalism into “an art form.” But Guilford took issue with Johnson’s willingness “to ham up the story, so there wasn’t much difference between news and entertainment ... He would write outrageous stories with only the slenderest connection of truth in them.”

Fletcher, the former Times editor, has compiled a list of Johnson’s greatest hits from Brussels: Johnson wrote that the EU wanted to standardize coffins, the smell of manure and the size of condoms — and had rejected an Italian request to make undersized rubbers. He warned Brits that their prawn-cocktail-flavored chips could be banned, that their sausages were under threat and that their fishermen would be required to wear hairnets.

Further goosing fears of the supranational state, Johnson wrote about the coming of compulsory European identification cards. (They are not coming.) He speculated that French, German and Dutch citizens would be elected to the British House of Commons. (Also not happening.) And he sought to underscore EU wastefulness with his description of a “kilometre-high Tower of Babel” to be built in Brussels.

James Landale, who worked with Johnson in Brussels and is the BBC’s diplomatic correspondent, wrote a poem to mark Johnson’s departure from the EU capital in 1995: “Boris told such dreadful lies / It made one gasp and stretch one’s eyes.”

Johnson has acknowledged that there was a bit of a game in all this.

“Everything I wrote from Brussels I found was sort of chucking these rocks over the garden wall,” he told BBC’s Desert Island Discs in 2005. “And I listened to this amazing crash from the greenhouse next door over in England. It really gave me this, I suppose, rather weird sense of power.”

Johnson and his spokesperson did not respond to the Post’s interview requests.

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Bill Newton Dunn, a long-serving British member of the European Parliament, said that, in person, Johnson was like a “puppy dog, anxious to please and get on with everybody.”

But less so in print.

“What was irritating is that he then started coming up with some extraordinary and, it turned out, completely inventive untrue stories about Brussels,” Newton Dunn said, recalling the headline: “Brussels recruits sniffers to ensure that Euro-manure smells the same.”

His former editor at the Telegraph, Max Hastings, has declared Johnson “unfit for national office.”

“There is room for debate about whether he is a scoundrel or mere rogue, but not much about his moral bankruptcy, rooted in a contempt for truth,” Hastings wrote in the Guardian.

Conrad Black, who once owned the Telegraph, fired back in the Spectator that Johnson is “more reliable and trustworthy” than Hastings.

Black had his own run-ins with Johnson, but he called his former charge “such an effective correspondent for us in Brussels that he greatly influenced British opinion on this country’s relations with Europe.”

“Boris’s peccadilloes were more absurd, complicated and over-publicized than the shambles of the personal lives of other journalists,” Black said. “But his editorial opinions were sensible and consistent. His shtick grew tiresome, like an overfamiliar vaudeville act, but he was at all times a person of goodwill and his foibles were deployed to the benefit of the enterprise.”

Sometimes, when Johnson’s words haven’t been supported by facts, people have excused him with the notion that he’s inattentive to detail. But Guilford emphasized that Johnson isn’t lazy: “He’s a workaholic. I’ve never seen someone so committed at the expense of everything.”

He recalled the two were at a mutual friend’s wedding in Ireland, and Johnson “spent every waking minute talking to Irish people” about their views on an upcoming abortion referendum.

“On Monday, we all woke up with a hangover to see this big centre page piece about the referendum. He spent the whole time doing it — irrespective of being there with friends and family,” Guilford said. “In my view, he never stops.”

Sonia Purnell, author of Just Boris: A Tale of Blond Ambition, was Johnson’s deputy in the Brussels office.

She told the Post that working alongside Johnson “wasn’t fun and it wasn’t productive. He’s very secretive, he’s really difficult to work with for that reason.” She said that he has a “frightening temper” and there’s “no team playing. He’s very much the solo performer.”

In her biography, she recalls Johnson launching into closed-door rants just before deadline, screaming obscenities at a potted plant, to lather himself up to write another scathing column.

To the Post, she characterized Johnson as ubercompetitive — suggesting that, going back to his childhood, he “competed at everything, including who was the blondest, who was the fastest, who was the cleverest.” In the adult Johnson, she said, that has resulted in a desire for “approval and love of the crowd” and an “incredible drive to be top dog.”

Purnell told the Guardian that she once said something to Johnson — made a joke about an official — in the kitchen of the Telegraph’s Brussels bureau. She saw her remarks in print a few days later. Johnson attributed them to “an EU source.”