Boris Johnson could face trial for misconduct in public after he claimed that the UK sends £350m to the European Union.

The private case is being brought against Johnson for allegedly abusing public trust during campaigning.

Marcus Ball, 29, is bringing the case, and claims Johnson endorsed and made statements which he knew to be false at a time when he was mayor of London and an MP.

A date has not yet been set for a court hearing, but could come after the Tory leadership contest has concluded – meaning Johnson, if he wins the race, may face trial as prime minister.

His first appearance at magistrates’ court could occur during the contest.

District Judge Margot Coleman had been asked to decide whether the Tory leadership candidate should appear in court over claims he was lying.

In 2016 the UK Statistics Authority found the £350m claim, which was plastered across an infamous Vote Leave campaign bus, to be a “clear misuse of official statistics”.

The slogan also claimed that the money could be used to fund the NHS instead.

Judge Coleman’s ruling said: “The allegations which have been made are unproven accusations and I do not make any findings of fact.

“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. The charges are indictable only.

“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. The charges can only be dealt with in the Crown Court.”

What does ‘misconduct in public office’ mean?

The Crown Prosecution Service sets out the offence as when a “public officer acting as such wilfully neglects to perform his duty and/or wilfully misconducts himself to such a degree as to amount to an abuse of the public’s trust in the office holder without reasonable excuse or justification.

The offence, which dates back to the 13th century, carries a maximum sentence of life imprisonment.

In the case against Johnson, the prosecution has to show evidence that the suspect is a ‘public officer’ and in this case, the fact that Johnson was London mayor and an MP at the time the claim was made, qualifies him as such.

According to the CPS, previous examples of misconduct in public office include: wilful excesses of official authority, ‘malicious’ exercises of official authority, wilful neglect of a public duty, and frauds and deceits.

She added: “I accept that the public offices held by Mr Johnson provide status but with that status comes influence and authority.”

A source close to Johnson said the prosecution was “nothing less than a politically motivated attempt to reverse Brexit and crush the will of the people”, the Press Association reports.

“The claimant has openly admitted that his plan is to overturn the referendum via a legal challenge and he clearly intends to try and undermine the one man who can truly deliver Brexit.

“The decision to issue a summons is extraordinary, and flies in the face of hundreds of years of British democratic tradition.”

Johnson’s position on the prosecution was outlined in the ruling, describing it as a “(political) stunt” and accusing Ball of trying to undermine the referendum result”.

Lewis Power QC, representing Ball, told Westminster Magistrates’ Court at a hearing last week: “Democracy demands responsible and honest leadership from those in public office.

“The conduct of the proposed defendant Boris Johnson was both irresponsible and dishonest. It was, we say, criminal.”

Tory Brexiteer Jacob Rees-Mogg said the decision was a “troubling” abuse of process.

“It is trying to use the courts to achieve a political end which, I think, is neither right or proper,” he told the Press Association.

“This is involving the courts in something that is not their area.

“We need courts and politicians to respect each other, and it is an abuse of process, and a troubling one.

“It has been brought by people who are resentful of the referendum result.”

Meanwhile, Tory leadership contender Michael Gove tweeted: “We should try not to criminalise free speech.”

Conservative former cabinet minister and barrister David Mellor said the ruling marked a “bad day for British justice”.

He told the Press Association: “This decision by a district judge right at the bottom of the judicial food chain is a deplorable absurdity.

“You really cannot have the courts adjudicating on what politicians (do) during election campaigns. There madness lies.

“I imagine there will be no shortage of senior judges who will feel acutely embarrassed about this. Politicians at election times exaggerate, and say things that may or may not be true, and it’s the electorate, not the courts, who should decide whether they are reliable or not.

“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. Is that what they really intended? Nutty, nutty, nutty.”

Ed Davey, expected to run the new leader of the Lib Dems, said in a statement released by the pro-EU group, Best for Britain that it is “only right” that Johnson is held accountable.

“Boris Johnson has never had to rely on these vital public services. That’s why he doesn’t care about the impact of his campaign slogans.

“Three years down the road and our NHS is on its knees. EU staff are leaving in their droves, having been made to feel unwelcome, while local hospitals around the country have warned that they won’t be able to deliver critical services in the event of a no-deal Brexit.

“We cannot let our communities down like this. Boris Johnson facing court is a sideshow – we need a final say vote on Brexit so that we can prevent the catastrophe he started.”

Ball has raised hundreds of thousands towards the case.

Solicitors Bankside Commercial are bringing prosecution on his behalf, and they have retained the services of Power and two other barristers from Church Court Chambers to handle the case.