Boris Johnson lied and engaged in criminal conduct when he repeatedly claimed during the 2016 EU referendum that the UK sent £350m a week to Brussels, lawyers for a crowdfunded private prosecution of the MP have told a court.

A legal team assembled by Marcus Ball, a 29-year-old businessman who has accused the former foreign secretary of misconduct in public office and raised more than £200,000 to finance the prosecution, laid out their case in front of a judge, who will rule next Wednesday on whether Johnson should appear in court.

The case concerned the “now infamous claim” by Johnson about the £350m, Lewis Power QC told Westminster magistrates court, and was not about preventing or delaying Brexit.

“It matters not whether you are Labour, Conservative or Liberal Democrat … or whether you are a leaver or a remainer,” he said. “Democracy demands responsible and honest leadership of those in public office. The conduct of the proposed defendant, Boris Johnson, was both irresponsible and dishonest. It was, we say, criminal.”

Power denied suggestions the case was a political stunt, adding: “The only political stunt were the lies told by the proposed defendant.”

Johnson was not present but a legal team assembled by him included Adrian Darbishire QC and lawyers from BCL solicitors. The offence of misconduct in public office carries a maximum sentence of life imprisonment.

“I should make it clear because of the interest in this case that it is absolutely denied by Mr Johnson that he acted in an improper or dishonest manner at any time,” said Darbishire.

Johnson, both as an MP and mayor of London at the time, chose to repeat “over and over, and over”, a claim he knew to be untrue, according to Power, who at one point quoted Winston Churchill on the duties of a member of parliament.

He said Johnson had repeatedly misrepresented the amount the UK sent to the EU a week by grossly inflating the figure.

“The identifiable problem is that when politicians lie, democracy dies,” said Power. “Justice and the rule of law is applied to all, regardless of their standing. Notoriety and fame offers Mr Johnson no protection to the rule that applies to us all.”

Referring to him by his full name, Alexander Boris de Pfeffel Johnson, Power said he “deliberately crafted” comments that would have consequences for those who heard them and was well aware of the implications.

Occasions when Johnson had made the claim included during an item on children’s television about Brexit, said Power, when Johnson stood with parliament behind him and said: “The reason you should be voting to leave is partly because it costs us a huge amount of money.”

For Johnson, Darbishire said the court had heard “a great deal of rhetoric and very little law”. There had never been a comparable case in common law, said the barrister, who said Ball’s case was based on a misunderstanding about the nature of misconduct in public office.

Johnson was not exercising state power when the claims were made, he said, adding that standing on the hustings in a contested political debate and making a campaign message “is not an exercise of the functions of government”.

“It doesn’t fall within the scope of this offence,” he said. “It was explicit in the early days of this process that if politicians could be prosecuted then it could assist in putting a halt to Brexit.”

He referred to a website associated with Ball listing Johnson as one of six potential “targets” in a supposed “rogue’s gallery”, which also included Dominic Cummings, the director of the Vote Leave campaign. Darbishire described the £350m figure as part of the communications strategy of Vote Leave.

“It’s a political motivation whichever way you look at it,” he said. “Today is just a culmination of that process with that motive running throughout.”

Reserving judgment over whether Johnson could be prosecuted under the misconduct charge, the judge, Margot Coleman, said she had been given a great deal to reflect on. She said she would provide her decision in writing to the parties on 29 May.

During the referendum campaign, the UK’s statistics watchdog delivered a public rebuke to the Vote Leave campaign for continuing to claim that EU membership cost the UK £350m a week.

A statement on the UK Statistics Authority’s website said the UK’s contribution to the EU was paid after the application of the UK rebate and that payments received by the British public and private sectors were also relevant.