Opponents argued that it was deliberately misleading and it became symbolic of the divisions caused by the referendum, which saw Britons vote by 52%-48% to leave the European Union.
District Judge Margot Coleman ruled that Johnson, a former British foreign secretary and ex-mayor of London, must answer a private summons alleging he had committed three criminal offences of misconduct in a public office.
In her written ruling at London’s Westminster Magistrates’ Court, Coleman said the accusations were not proven.
But she added: “Having considered all the relevant factors I am satisfied that this is a proper case to issue the summons as requested for the three offences as drafted.
“This means the proposed defendant will be required to attend this court for a preliminary hearing, and the case will then be sent to the Crown Court for trial,” she said.
Johnson’s lawyers argued the case was no more than a stunt by those opposed to Brexit and an attempt to use criminal law to regulate the content of political debate for the first time in English legal history.
“The decision to summon Boris Johnson is extraordinary. It is not the role of criminal law to regulate political speech,” said an unnamed source close to Johnson, according to quotes cited by the Daily Mail newspaper’s deputy political editor.
“This runs counter to centuries of British political tradition and risks undermining our democracy.”
Polls suggest that the flamboyant politician, known for his tousled blond hair, is well ahead of the other 10 declared candidates in the contest to replace May as leader of Conservative Party and prime minister.
He was one of the leading supporters of Brexit during the 2016 vote and quit the government over May’s EU divorce deal which parliament rejected three times, leading her to announce she would step down as party leader on June 7.
Misconduct in a public office carries a maximum sentence of life in prison and if he were found guilty, it could spell the end of Johnson’s political career.
Critics said the case itself showed he was unfit to be prime minister although other commentators suggested it could boost his leadership campaign by burnishing the anti-establishment image the Eton and Oxford-educated Johnson has cultivated.
The summons was sought by “Brexit Justice”, a crowdfunded group seeking a private prosecution against Johnson, arguing he deliberately made false comments about the cost of Britain’s EU membership before both the referendum and the 2017 national election.
“During both time periods outlined above, the (proposed) defendant repeatedly lied and misled the British public as to the cost of EU membership, expressly stating, endorsing or inferring that the cost of EU membership was 350 million pounds ($442 million) per week,” its application said.
In submissions to the court, Johnson’s lawyers said the case was brought for purely political purposes to undermine Brexit.
“Its true purpose is not that it should succeed, but that it should be made at all,” his lawyer said. “And made with as much public fanfare as the prosecution can engender.”
In September 2017, government statisticians criticised Johnson for repeating the 350 million pound figure which he argued could be spent on public services in Britain. They said it ignored the rebate that Britain received from the EU.
“It is a clear misuse of official statistics,” David Norgrove, chairman of the UK Statistics Authority, then wrote in a letter to Johnson.
Figures published by the Office for National Statistics in April 2016 showed Britain’s weekly net contribution to the EU to be about 190 million pounds a week.