THE Mayor of London, Boris Johnson, could face a High Court investigation over his involvement in the decision by Transport for London (TfL) to ban an advertising campaign by the ex-gay group Core Issues Trust.

The Trust had arranged for adverts to be displayed on buses in April 2012 reading "Not gay! Ex-gay, post-gay and proud, get over it", as a response to a similar campaign by the gay-rights group Stonewall.

Four days before the adverts were due to appear, The Guardian ran a story on its website, resulting in significant levels of criticism on the website, on social-media sites, and direct to the Mayor's office.

Within hours, the advert had been pulled, and the court heard that the Mayor's advisers briefed the press that the decision had been taken by Mr Johnson, who was "strongly of the view that the ad should not run".

The Master of the Rolls, Lord Dyson, ruled in the Court of Appeal on Monday that TfL would have been right to ban the adverts if the decision to do so was based on their advertising policy; but he said that it would have been "unlawful" if the decision had been taken "to further the mayor's political campaign".

At last year's High Court hearing, TfL argued that the decision was taken by its communications director rather than the Mayor. The court agreed, and ruled that the decision was lawful. New evidence has since emerged through a Freedom of Information Act request, which, Lord Dyson said, "raises a strong prima facie case" that the decision was taken by Mr Johnson.

"There is now in evidence an email which...states that the Mayor instructed TfL to pull the advertisement," Lord Dyson said.

The Court of Appeal ordered that the case should be re-heard by the High Court, and that Mr Johnson, together with his deputy, and his external-affairs director, should all be ordered to give written evidence before the judge decides whether they should be cross examined.

The fresh High Court hearing will decide only whether the decision was taken lawfully. The Court of Appeal made it clear that TfL could have reached the same outcome if it had followed proper procedures.

Stonewall advert hearing. A separate hearing will be held to determine whether TfL were right to accept the Stonewall advert. The Core Issues Trust had sought an injunction after it was announced that Stonewall was to re-run the adverts last year (News, 1 November).

The Trust argued that the High Court had decided that the Stonewall poster was also in breach of the TfL advertising policy; but this claim was rejected by both Stonewall and TfL. In Monday's judgment, Lord Dyson said that the High Court judge "did decide that the Stonewall advertisement failed to comply with the policy".

Lord Justice Briggs said that Stonewall's "Get over it" slogan was "a confrontational message which is likely to come across to many...as at least disrespectful of their sincerely held beliefs, and, to some, as suggesting that there is no place for the toleration of their beliefs in modern society".