London (AFP) - Boris Johnson, the frontrunner to become Britain's next prime minister, must attend court over allegations that he knowingly lied during the Brexit referendum campaign, a judge announced Wednesday.

Johnson, the former foreign secretary, will be summoned to appear before a court over allegations of misconduct in public office, judge Margot Coleman said in a written decision, without specifying the date.

The case is over the claim that Britain sends Â£350 million ($440 million, 400 million euros) a week to the European Union.

The exact amount of Britain's net and gross EU contributions was one of the biggest issues during the 2016 referendum campaign.

Businessmen Marcus Ball has crowd-funded the private prosecution.

Coleman's decision follows a hearing last week at Westminster Magistrates Court in London.

"The allegations which have been made are unproven accusations and I do not make any findings of fact," the judge said.

"Having considered all the relevant factors I am satisfied that this is a proper case to issue the summons as requested," she wrote.

Referring to Johnson as the "proposed defendant", Coleman said he would be required to attend a preliminary hearing that could then result in a trial.

- Johnson 'repeatedly lied' -

Johnson is considered the favourite among the 11 candidates vying to replace the outgoing Theresa May as leader of the governing Conservative Party, and therefore prime minister, by the end of July.

The summons application made by Ball's lawyers alleges that Johnson "repeatedly lied and misled the British public as to the cost of EU membership" and "knew that such comments were false or misleading".

"Lying on a national and international platform undermines public confidence in politics."

The maximum penalty for misconduct in public office is life imprisonment.

The elements of the offence are that a public officer, acting as such, wilfully neglects to perform their duty or wilfully commits misconduct to such a degree as to amount to an abuse of the public's trust in the officeholder, and does so without reasonable excuse or justification.

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Johnson was not present at last week's hearing, but his lawyer Adrian Darbishire said the pro-Brexit figurehead staunchly denied acting in an improper or dishonest manner.

- 'Unusual and exceptional' -

Coleman considered whether the case had been brought purely to annoy Johnson.

The former London mayor's position is that the application is a politically-motivated stunt as part of a campaign to undermine the referendum result, and/or to prevent its consequences.

Britain is due to leave the EU on October 31.

"I accept the defence submission that when the applicant commenced his consideration of whether to bring a private prosecution against the proposed defendant, some three years ago, there may have been a political purpose to these proceedings," Coleman said.

"However, the information for the summons was laid on February 28, 2019 and that argument in my view is no longer pertinent.

"I do not accept the application is vexatious."

In her 14-page decision, the judge said: "This is an unusual and exceptional application with a considerable public interest and it is right that full reasons are provided."