Media playback is unsupported on your device Media caption Marcus Ball, left, is bringing a private prosecution against Boris Johnson

Boris Johnson has been ordered to appear in court over claims he lied by saying the UK gave the EU £350m a week.

The Tory leadership candidate has been accused of misconduct in public office after making the claim during the 2016 EU referendum campaign.

It is a private prosecution launched by campaigner Marcus Ball, who crowdfunded £200,000 for the case.

A source close to Mr Johnson called the case a "politically motivated attempt to reverse Brexit".

His lawyers argued it was "a stunt".

The preliminary hearing will take place at Westminster Magistrates' Court and the case will then be sent to the Crown Court for trial.

The BBC's assistant political editor, Norman Smith, said the allegations could not come at a worse time for Mr Johnson, and his critics are likely to use the claims against him in the upcoming contest to become next Tory leader and prime minister.

The £350m figure was used by the pro-Brexit Vote Leave group throughout the referendum. It also appeared on the side of the campaign bus, which urged the UK to "fund our NHS instead".

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The former foreign secretary faces three allegations of misconduct in public office, between 21 February and 23 June 2016, and between 18 April and 3 May 2017.

The first period covers the time in which he campaigned for a Leave vote in the EU referendum, while the second covers the general election campaign that year.

Mr Ball's lawyers lodged an application in February to summons Mr Johnson, claiming that while an MP and mayor of London, he deliberately misled the public during the first campaign, and repeated the statement during the second.

Lewis Power QC, who represents Mr Ball, said Mr Johnson's conduct had been "both irresponsible and dishonest".

"Democracy demands responsible and honest leadership from those in public office," he said.

'Infamous statement'

Mr Power said the prosecution's application was not brought to undermine the result of the 2016 referendum and it was not about what could have been done with the saved money.

"The allegation with which this prosecution is concerned, put simply, is Mr Johnson repeatedly misrepresented the amount that the UK sends to Europe each week," he said.

"It is concerned with one infamous statement: 'We send the EU £350m a week.'

"The UK has never sent, given or provided £350m a week to Europe - that statement is simply not ambiguous."

What is misconduct in public office?

It's an ancient offence with roots back to the 13th Century

It can only be brought against someone who is exercising some kind of official function - such as a civil servant, a prison officer or someone else entrusted to carry out a public role

Someone is guilty of the offence if a prosecution can prove that the official wilfully neglected to perform their duty - or "misconducts" themselves - to such a degree that it amounts to an abuse of the public's trust in the office holder

The offence can lead to life imprisonment

The source close to Mr Johnson said the decision to summon him was "extraordinary" and "risks undermining our democracy".

"It is not the role of criminal law to regulate political speech," they said.

"If this case is allowed to proceed then the state, rather than the public, will be put in charge of determining the strength of arguments at elections."

The private prosecution was also criticised by Conservative MP Jacob Rees-Mogg, who said it was "fundamentally ill-judged and improper".

"It is a grave error to try and use legal process to settle political questions," he told the BBC.

"The issue at hand is whether it was right to use the gross or net level of our contribution to the European Union - that is a matter of free speech and the democratic process."

But in her written ruling, District Judge Margot Coleman said: "The applicant's case is there is ample evidence that the proposed defendant knew that the statements were false."

She continued: "I accept that the public offices held by Mr Johnson provide status, but with that status comes influence and authority."

She added that there was sufficient evidence of an issue to proceed with a trial, though she stressed the allegations were currently unproven.

How does a private prosecution work?

Anyone can bring a private prosecution - for example, the RSPCA pursue animal cruelty cases all the time.

But the Director of Public Prosecutions, Max Hill QC, has the power to either take over a case or stop it in its tracks.

He can therefore authorise the Crown Prosecution Service to take over Mr Johnson's case if the allegations made by Mr Ball pass the CPS' own evidence test, there is a public interest in doing so, or there is a particular need to get involved.

The DPP can stop a private prosecution if he concludes it's vexatious, malicious or flawed for a range of other reasons.

Given the district judge has already declared there is a prima facie case to be tested, the DPP may decide there's nothing he can usefully add and stay well clear.

So Mr Johnson's lawyers may already be looking at how to challenge the decision's lawfulness. And that could mean the case - like many other private prosecutions - becomes mired in legal argument and appeals.