Mr. Johnson, wild-haired and witty, became a chief architect of Brexit when, four months before the referendum, he threw his weight behind a cause until then most closely associated with the populist U.K. Independence Party. But his main contribution to Brexit may go back more than two decades.

A correspondent in Brussels for The Daily Telegraph in the early 1990s, Mr. Johnson was credited by fellow reporters with pioneering the euroskeptic coverage of the European Union that has since become the default setting for much of the British press. With little regard for the truth — he was previously fired by The Times of London for making up a quote — Mr. Johnson wrote about a Europe scheming to impose standard condom sizes and ban his country’s beloved prawn-cocktail-flavored chips (both untrue).

“Boris invented fake news,” said Martin Fletcher, a former foreign editor of The Times, who was in Brussels shortly after Mr. Johnson. “He turned euroskepticism into an art form that every news editor in London came to expect.”

Before the referendum, Mr. Fletcher added, “Boris campaigned against the cartoon caricature of Brussels that he himself invented.”

The campaign was marked by a relentless drip of anti-immigration rhetoric and a couple of big lies that stuck: the 350 million pounds (about $450 million at current rates) that Britain paid to the European Union every week (false) and the prospect of millions of Turks’ making their way to Britain if it stayed in the union (Turkey is not joining the bloc). Two years ago, the United Nations urged Britain to deal with hate speech in its newspapers, specifically citing a column in The Sun that compared migrants to cockroaches and the norovirus.

The tabloids say they merely reflect the concerns and fears of their readers. But their critics say they poison the debate by playing to people’s worst instincts and prejudices, distorting facts and creating a propaganda ramp that mainstreams intolerance and shapes policy.