A Christian group is challenging Boris Johnson’s decision not to allow their adverts for “curing homosexuality” to run on buses in the capital.

On Thursday, Dr Mike Davidson of the Core Issues Trust will appear in the High Court and argue that the Mayor of London’s decision to pull the adverts created by a duo of Christian groups was unlawful stifling of their free speech.

Although the ad campaign was passed by the Committee of Advertising Practice, Mr Johnson intervened to pull it before it could run – just days before the mayoral election in May 2012.

The posters said: “Not Gay. Ex-Gay, Post-Gay and Proud. Get over it!” and were in response to a previous poster campaign by Stonewall, the gay rights group, which said: “Some people are gay. Get over it!”

Mr Johnson is responsible for approving advertising on public transport in London.

At the time of his intervention, Mr Johnson said: “London is one of the most tolerant cities in the world and intolerant of intolerance. It is clearly offensive to suggest that being gay is an illness that someone recovers from and I am not prepared to have that suggestion driven around London on our buses.”

Ahead of the hearing, Dr Mike Davidson said: “This is all about being free to talk about these issues. It was a mistake to assume these views we were expressing came from entrenched homophobia, and failed to recognise that people who want to walk away from their homosexual feelings are a group in their own right.”

The trust will draw attention to a poster which some Christians found offensive. Funded by Richard Dawkins, the academic, and the British Humanist Association in 2009, it said: “There’s probably no God. Now stop worrying. And enjoy your life.”