LONDON (AFP) - Former foreign minister Boris Johnson, the front runner to replace Mrs Theresa May as Britain's prime minister, on Friday (June 7) won his bid to block a lawsuit accusing him of knowingly lying during the Brexit referendum campaign.

The decision removes a hurdle for Mr Johnson in his leadership bid, with his lawyers persuading a London court that the private prosecution was "politically motivated and vexatious".

They asked the High Court to throw out a judge's decision last month to allow a summons ordering Mr Johnson to appear in court over allegations of misconduct in public office, arguing the decision had "erred in law".

"We are quashing the decision of the district judge to issue the summonses," Judge Michael Supperstone, one of two High Court judges hearing the case, said following a hearing on Friday, at which Mr Johnson was not present.

The case, brought by businessman Marcus Ball in a crowd-funded initiative, concerned Mr Johnson's claim that Britain sends £350 million (S$608.27 million) a week to the European Union.

While this was Britain's gross contribution, the net figure accounts for a budget rebate from the EU as well as payments to Britain's public sector from the EU Budget, and is substantially less.

The official Leave campaign emblazoned the controversial figure on the side of its touring bus during the 2016 EU referendum, while Mr Johnson and other Brexiteers repeatedly trumpeted it campaigning.

Mr Ball, 29, who has crowdfunded more than £300,000 through an online campaign to bring the case, told reporters ahead of the hearing he believed in "the merits of it".

In a written ruling on May 29, district judge Margot Coleman had agreed that there was a proper case to issue a summons.

But Mr Adrian Darbishire, Mr Johnson's lawyer, asked the High Court on Friday to throw out the prosecution because it was political in nature.

"The only rational conclusion which could be reached was that the prosecution was politically motivated and, therefore, vexatious," he said.

The two-judge panel agreed.

"It was the conclusion of the court that we were persuaded by Mr Darbishire," said judge Anne Rafferty.

"This quashes the summons," she said on revealing their ruling.

Mr Johnson on Monday launched his campaign to succeed Mrs May as Conservative leader.

She steps down on Friday and formally triggers the race for a successor - currently being contested by 11 MPs, including Mr Johnson - but will remain prime minister until a new leader is chosen, likely in late July.