Iain Duncan Smith has described the court case in which Boris Johnson will face accusations of misconduct in public office as “ludicrous”.
Mr Johnson faces a private prosecution over claims he was lying when he said the UK gave the EU £350 million a week during the 2016 EU referendum.
Mr Duncan Smith told talkRADIO’s Julia Hartley-Brewer: “It is ludicrous in practical terms but also it is bizarre to bring the courts into what is essentially a debating point.
“I think the district judge is grandstanding.”
Campaigner Marcus Ball lodged an application to summons Mr Johnson to court, claiming he had deliberately misled the public during the Brexit referendum campaign in 2016 and then repeated the statement during the 2017 general election.
However, Mr Duncan Smith said that he still “believed” in the £350 million figure.
“Boris Johnson – alone – is now apparently a liar. Michael Gove is serving in cabinet, what about him?,” the former Conservative leader said.
“What about me? I was on that team and I was very happy with that figure. I debated it endlessly and I am still happy to debate it. I still believe in it.”
Mr Duncan Smith said the case against the former foreign secretary was “all about the leadership election”.
“Remainers fear Boris Johnson anywhere near leadership,” he added.
“It is a political issue that has now been sucked into the courts.”
In a summary for the court, Mr Johnson described the claim as "a political stunt".
His position in summary said: "This application is brought for political purposes. The position presented to the Court is that this is a disinterested attempt to improve the standards of political debate."
It added: "The application is a (political) stunt. Its true purpose is not that it should succeed, but that it should be made at all. And made with as much public fanfare as the prosecution can engender."