Boris Johnson, the favourite to replace Theresa May as British prime minister, must appear in court over allegations he lied about Brexit, a judge has ruled.
Johnson stated Britain would be STG350 million ($A640 million) a week better off outside the EU in the lead-up to the 2016 Brexit referendum.
Famously emblazoned on a campaign bus, the number was a central and controversial part of the Leave campaign's successful "take back control" message.
Opponents argued it was deliberately misleading and it became symbolic of the divisions caused by the referendum, which saw Britons vote by 52-48 per cent to leave the EU.
District Judge Margot Coleman ruled on Wednesday 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 there was a case.
"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," she wrote.
"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."
Johnson's spokesman was not immediately available for comment, but his lawyers had 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.
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 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.
In submissions to the court, Johnson's lawyers said the case was brought for purely political purposes.
"Brexit Justice Limited is the product of a campaign to undermine the result of the Brexit referendum, and/or to prevent its consequences," his lawyer argued.
In September 2017, government statisticians criticised Johnson for repeating the STG350 million 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," UK Statistics Authority chairman David Norgrove 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 STG190 million ($A347 million) a week.
