LONDON—The frontrunner to become Britain’s next prime minister this summer has been summoned to court to face allegations of misconduct in public office over alleged lies told during the Brexit campaign.

In an unprecedented ruling issued here on Wednesday, a judge has paved the way for Boris Johnson to stand trial for a false claim that was at the very center of the Brexit campaign. The Vote Leave campaign bus was emblazoned with the slogan: “We send the EU £350m a week.” The independent U.K. Statistics Authority said the figure was misleading and the Institute for Fiscal Studies argued it was “absurd,” and yet Vote Leave, and Johnson in particular, continued to use the number throughout the hotly contested Brexit referendum in 2016.

One anti-Brexit campaigner was so aggrieved by the prominent use of the claim—which became a symbol of “fake news” during and after the bitter campaign—that he has crowdfunded a legal challenge.

To the shock of the British political establishment, a judge agreed Wednesday that Johnson, the former Conservative mayor of London, should face trial for deliberately lying to the public. “It is alleged that the conduct of which the proposed defendant is accused was a huge lie calculated to mislead the electorate by using inaccurate and misleading statements,” District Judge Margot Coleman wrote in her written statement.

The timing of this extraordinary case could scarcely be worse for Johnson, coming less than a week after he confirmed that he would run to succeed Theresa May as leader of the Conservative Party and thus British prime minister.

Although ardent Brexiteers are likely to rally to Johnson’s side, it will reinforce the notion that the controversial—at times, bumbling—character seems to attract unnecessary trouble. Around 12 Conservative lawmakers are expected to take part in the leadership race, which will be whittled down to two candidates by Conservative members of Parliament before the finalists are put to a vote of party members. Johnson is extremely popular with the grassroots members.

Even before this latest hiccup, many of his colleagues in Parliament were engaged in a “Stop Boris” campaign as they wanted to ensure that safer pairs of hands were put forward to the vote of the members.

The judge in London said the campaigner Marcus Ball, who raised $250,000 to bring the case, had provided “ample evidence” that Johnson was knowingly misleading the public. For example, Johnson said Britain sent the EU around £10 ($12) billion per year during a TV interview in May 2016, which was around half of the controversial £350 ($450) million a week figure.

“I accept that the public offices held by Mr. Johnson provide status but with that status comes influence and authority,” she wrote. “[Ball’s lawyer] submits there will be seldom a more serious misconduct allegation against a member of Parliament or mayor than to lie repeatedly to the voting public on a national and international platform, in order to win your desired outcome. I am satisfied this element of the offense is prima facie satisfied.”

This statement implies no guilt on Johnson’s part, merely that the threshold for trial has been met. “The allegations which have been made are unproven accusations and I do not make any findings of fact,” she said.

Johnson has not yet responded to the ruling but his legal argument submitted to the court argued that the private prosecution was no more than a political “stunt.”

“This application is brought for political purposes,” said Adrian Darbishire QC, representing Johnson.

He said the claim was based on public information and was properly scrutinized throughout the campaign. “As with very many claims made in political campaigns, it was challenged, contradicted, and criticized,” he said.

None of Johnson’s leadership rivals have issued statements after the bombshell ruling, which is likely to become a divisive issue in the campaign.

Former Conservative cabinet minister and barrister David Mellor said the ruling was a “deplorable absurdity.”

“You really cannot have the courts adjudicating on what politicians [do] during election campaigns. There madness lies,” he told the Press Association, suggesting there may be a backlash in favor of Johnson.

“This is a bad day for British justice. But probably, contrary to the wishes of those who have crowdfunded this nonsense, a big boost to Boris.”