Now private prosecutor Marcus Ball has notified the former Foreign Secretary of his intention to bring a court case over alleged misconduct in public office.
Backed by around £300,000 raised through crowdfunding appeals, and with the support of a team of lawyers, he now intends to lay an information at a magistrates court early in the new year.
It is understood magistrates would then have to determine whether to allow the case to go ahead.
Mr Ball alleges the claim, made or endorsed by Mr Johnson during the 2016 referendum campaign, that the UK ‘sends £350 million a week to the EU’ was knowingly false and made with the intention to sway votes.
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He has spent two years preparing the case and raising finance, and has instructed solicitors Bankside Commercial to bring a prosecution on his behalf.
They have retained the services of Lewis Power QC and two other barristers from Church Court Chambers to handle the case.
Asked if he is mounting the case in a bid to prevent Britain leaving the EU, Mr Ball said: ‘This is about stopping lying in politics; this is not about stopping Brexit, the courts do not have the power to do that.
‘We do need to stop politicians from lying in any future referendums though. The motivation for this prosecution is a desire to bring a beginning to the end of lying in politics.’
Having raised £286,000 from around 7,000 supporters in a series of appeals, Mr Ball is now seeking to crowdfund a further £500,000 to meet the costs of the case as it proceeds. His Crowdfunder website showed £87,000 of this had been donated by Thursday.
A spokesman for Mr Johnson declined to comment.
The former Foreign Secretary has repeatedly stood by the £350 million figure used by the Vote Leave campaign during the referendum, which he says represented the UK’s weekly gross contribution to EU budgets.
In January, he told The Guardian Vote Leave ‘grossly underestimated the sum over which we would be able to take back control’, arguing the gross figure would reach £438 million by the end of a post-Brexit transition period.
Mr Power said Mr Ball’s legal team are ‘duty bound not to comment upon the guilt or innocence of Mr Johnson as this is a decision that can only be made within a court of law’.