A High Court judge declared yesterday that she was "not satisfied that the full story is being told" regarding Boris Johnson's 2012 intervention to stop ex-gay adverts running on London buses.
Ms Justice Lang demanded that the London Mayor's office and Transport for London provide "full and unredacted" copies of all internal documentation and communications related to the ban of the posters "in order to get to the bottom of this".
She also required that Johnson make a written statement about his involvement in the decision to pull the adverts, which she would form the basis of her decision over whether he should give evidence under oath in court.
Core Issues Trust, a charity involved with assisting people with unwanted same-sex attraction, wanted to run posters on London buses with the slogan: "Not Gay! Ex-Gay, Post-Gay and Proud. Get over it!"
This was a response to adverts bearing the tagline, "Some People Are Gay. Get Over it!", gay lobby group Stonewall.
Although it was claimed that the ban of the trust's posters was linked to TfL's advertising policy, it was later revealed that the Mayor of London had directly intervened.
This prompted the trust to take legal action, arguing this was an abuse of Mr Johnson's mayoral powers.
While an initial court proceeding ruled in favour of Mr Johnson, a later Freedom of Information Act request brought to light e-mails that suggest Mr Johnson's actions were politically motivated.
In late January 2014 the Appeals Court heard transcripts of an internal email from Deputy Mayor Richard Barnes on the day that the decision to censor the posters was made, which said: "I believe that we should take a strong and immediate line on this and get them stopped."
In an email sent minutes later, Guto Harri, the Director of External Affairs of the Mayor's office said this in an email: "Boris has just instructed TfL to pull the adverts and I've briefed the Guardian, who will break that news in next half hour."
In the Appeal court's ruling, the second most senior judge in England and Wales, Master of the Rolls, Sir John Dyson, said: "There is now in evidence an email which unequivocally states that the Mayor instructed TfL to pull the advertisement.
"[The email] shows that the Mayor's office contacted the Guardian immediately in order to make political capital out of the story ... [as] arrangements had been made for the Mayor to appear ... at hustings organised by Stonewall [the following day].
"This is a most unsatisfactory state of affairs ... If the Mayor took the decision, the question arises what his motives were."
There are strong concerns that Mr Johnson was politically motivated due to the mayoral elections which were held three weeks after the decision to censor the trust's adverts.
The trust has expressed frustration that Stonewall has been permitted to continue advertising, despite the fact that the Master of the Rolls held in his judgement that posters by both organisations were in breach of TfL's advertising policy.
Andrea Williams, chief executive of the Christian Legal Centre, said: "TfL's continued promotion of Stonewall campaigns on its transport system is highly provocative and shows disregard for the Court's judgment."
Speaking about Ms Justice Lang's decision, Ms Williams said "This 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."