A High Court judge is this week investigating accusations that Boris Johnson banned a bus ad from an ‘ex-gay’ group for political reasons.

The court is investigating claims that the London Mayor was politically motivated in his decision to ban an ad in April 2012, due to his re-election campaign at the time.

The advert, paid for by the Christian charity Core Issues Trust, read: “Not Gay! Ex-gay, post-gay and proud. Get over it!”.

The High Court initially upheld the ban last year, but in January the Court of Appeal ruled that the case must be re-opened, after internal emails from the mayor’s team came to light.

The High Court’s initial judgement was made on the basis that the decision came from within Transport for London, but an email from Johnson’s media chief at the time of the ban read: “Boris has just instructed TfL to pull the adverts and I’ve briefed the Guardian, who will break that news in next half hour.”

Christian Concern claim: “This is suppression of free speech and expression by the political class. Boris and his high-level team are using their power to suppress a small Christian charity.

“The fact that the Court of Appeal ordered this case to be reconsidered by Mrs Justice Lang is an important vindication of the rule of law.

“TfL has made it hard for us to get to this point; it has been hostile and obstructive and has certainly not been a model of transparency.

“TfL has continued to promote Stonewall campaigns on its transport system – a highly provocative action which shows disregard for the Court’s original judgment, which held that neither Stonewall’s or Core Issues Trust’s adverts should have been allowed.

“If one point of view is championed on London’s transport network, there must be room to display an opposing view. We hope the Judge will recognise that this freedom was violated when the Mayor banned Core Issues Trust’s adverts.”

Nigel Pleming QC, representing the Mayor and Transport for London, rejects the accusations that the move was politically motivated.

Following the two-day hearing, Mrs Justice Lang said she would need time to reach her verdict in the case.