A Christian legal group challenging the ban on an ad campaign for controversial gay cure therapies, upheld by the High Court this month, has demanded memos from Boris Johnson in a new bid to prove he acted unlawfully.

The Telegraph reports that the Christian Legal Centre has demanded that the Mayor of London, Boris Johnson, release internal memos which they claim will show he pressured officials to ban an advertising campaign for ‘gay conversion’ therapy.

Posters for the campaign, which were due to run on 24 buses, said: “Not Gay. Ex-Gay, Post-Gay and Proud. Get over it!” and were in response to a previous poster campaign by gay rights charity Stonewall, which said: “Some people are gay. Get over it!”

Boris Johnson, whose role as Mayor of London puts him in charge of Transport for London (TfL), intervened to block the adverts in May 2012.

Last month, Dr Mike Davidson of Core Issues Trust, an anti-gay Christian group, told the High Court that TfL’s decision to pull the adverts was unlawful and “a deep threat” to democratic freedom.

However, on 22 March Mrs Justice Lang of the High Court backed the ban, saying the ads would raise “the risk of prejudice and homophobic attacks” if released.

She qualified the ruling by noting that TfL’s  process in introducing the ban “was procedurally unfair, in breach of its own procedures and demonstrated a failure to consider the relevant issues”. She noted that there seemed to be “missing” evidence as to how TfL had decided on the ban.

The Christian Legal Centre has now claimed the missing evidence is part of a “cover up”. It has demanded the release of internal memos under the Freedom of Information Act.

Andrea Williams, the Centre’s director, said: “Boris is increasingly under pressure for his actions and integrity. This is another example of where he has been expedient as opposed to doing truth and justice.”

Last November, Lib Dem Health Minister Norman Lamb said of gay conversion therapy: “There is no evidence that this sort of treatment is beneficial and indeed it may well cause significant harm, to some patients.”