London Mayor Boris Johnson blocked the Core Issues Trust's bus posters last April, saying they were 'offensive to gays'

Christian groups are complaining of "censorship" after the High Court upheld the Mayor of London's ban on a controversial bus advert suggesting gays can be helped to "move out of homosexuality".

A judge rejected accusations by Core Issues Trust, a Christian charity behind the ad, that Boris Johnson used his position as chairman of Transport for London (TfL) to obtain the ban in order to secure the gay vote and advance his 2012 re-election campaign.

Mrs Justice Lang, sitting at London's High Court, said: "In my view, such unlawfulness has not been established on the evidence."

But she gave the Trust permission to appeal because of the fundamental issues raised by the case over the right to freedom of expression. The ad posters earmarked for the sides of the capital's buses read: "Not Gay! Ex-Gay, Post-Gay and Proud. Get over it!"

The trust said they were in response to a bus poster campaign by gay rights group Stonewall, which carried the message: "Some people are gay. Get over it!"

Mr Johnson condemned the "gay cure" ad as "offensive to gays" and said it could lead to retaliation against the wider Christian community.

The trust, which says its works with gay people seeking to change their lifestyles but rejects the idea of offering a gay cure, argued its right to freedom of expression under Article 10 of the European Convention on Human Rights had been violated by the ban.

The judge ruled the trust had been a victim of unfairness in the way the ban had been introduced in April last year. She said: "TfL's decision-making process fell below the standards to be expected of a responsible public body."

But the Trust's Article 10 rights were outweighed by the rights of gays to respect for their private and family life under article 8(1). The judge said TfL was legally justified in imposing the prohibition in the run-up to the mayoral elections in May 2012 because the ad would cause "grave offence" to gays and "increase the risk of prejudice and homophobic attacks."

At the same time TfL's interests "coincided with those of Mr Johnson, who also wished to avoid causing offence and avoid criticism which might damage his election campaign," and the judge said: "The overlap in interests did not render the decision unlawful."