London Mayor Boris Johnson is to be investigated over his intervention to prevent an "ex-gay" advert being displayed on London buses.
A court ruled today that Johnson's role in stopping Transport for London from accepting the advert must be investigated by the High Court.
The advert was placed by the Christian Legal Centre and Core Issues Trust and featured the strapline, "Not Gay! Ex-Gay, Post-Gay and Proud. Get over it!".
It was a response to ads previously run on London buses by gay lobby group Stonewall, reading "Some People are Gay. Get over it!"
The Christian Legal Centre challenged Johnson's intervention on the grounds that it was politically motivated.
Such an intervention by Johnson would be a serious overreach of his political authority, and a demonstration of favouritism to Stonewall, whose adverts ran without interference.
The court ruling on the case today heard evidence in relation to an email from Guto Harri, the Mayor's Communications Director at the time, which 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."
The second most senior judge in England and Wales, Master of the Rolls, Sir John Dyson, said in his ruling: "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 ... 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."
In its response, the Christian Legal Centre hailed a "significant victory".
"The courts have fulfilled an historic duty of holding politicians to account and refusing to tolerate non transparent behaviour," it said.
"The decision of the court is one of the most significant in recent times in ensuring that powers are exercised by politicians properly."
Previous hearings in March last year had gone ahead without access to the email evidence. In her ruling at the time, Ms Justice Lang had described the removal of the adverts as "procedurally unfair, in breach of [Transport For London's] own procedures and demonstrated a failure to consider the relevant issues".
However, she considered the banning of the advert as legitimate because it would "cause grave offence" to the gay community.
She also listed other advertisements, such as Stonewall and the British Humanist Association's "There's probably no God" poster - as breaching TfL's policy and being equally deserving of a ban.
However, Stonewall has continued its advertisements without hindrance, ignoring the initial ruling which has now been affirmed by Sir John.
He stated that Justice Lang was entitled to rule on the matter of the Stonewall advertisement, and that her conclusion was accurate also. In his judgement, he agreed that the "the Stonewall advertisement had failed to comply with the [TfL] policy".
Andrea Minichiello Williams, chief executive of the Christian Legal Centre, said: "The effect of such a ruling means that the past and present campaign being run by Stonewall breaches Transport for London's Advertising standards and is illegal. Stonewall knows this but has brazenly decided to pursue its agenda.
"In a mature democracy both sides of a debate should be heard but it seems that Boris Johnson, Transport for London and Stonewall are intent to shut down the Christian side of the debate by fair means or foul.
"It is a great relief that the Master of the Rolls has ruled to hold to account arbitrary use of the exercise of power by a public authority."
Director of the Core Issues Trust, Dr Mike Davidson, has written a letter calling on Johnson to make all e-mail records, current, past, and previously deleted, available to the trust's lawyers.
"As I am sure you will be aware, the political credibility of politicians is at an all-time low ... Your openness, transparency and integrity in helping with pre-court access to these matters will be much appreciated."
He added: "If such access is not offered, our lawyers will petition Mrs Justice Lang to order such access as part of further enquiry as the Master of the Rolls has so directed."
However, Sir John rejected arguments from the Christian Legal Centre that the removal of the advertisements amounted to a breach of freedom of religion.
He also dismissed suggestions that a "right to buy" and "right to reply" permitted Core Issues Trust to display the posters in response.
He took the position that while Stonewall's adverts were intended to promote tolerance, that fact in itself did not make them respectful to people of faith.
He also included an affirmation that ex-gays are protected by the Equality Act 2010 as any discrimination against them is "because of ... sexual orientation" and that it is not relevant whether the discrimination is based on past or current sexuality.