The Mayor of London, Boris Johnson, has blocked a bus advert looking to counter similar gay activist ads successfully released in recent years. The adverts in question looked to support those who profess to be "ex-gay" or "post-gay" from criticism by gay activists.
The advert featured the slogan: "Not gay! Ex-gay, Post-gay and proud. Get over it!"
It had been due to appear on London buses along five routes for two weeks, starting Monday.
The ad had previously been cleared by advertising regulators and given the go-ahead, however, London Mayor Boris Johnson weighed in on the issue, using his position as chairman of Transport for London to ban it from London's buses.
Johnson defended his actions saying that London should be "intolerant of intolerance."
"It is clearly offensive to suggest that being gay is an illness that someone recovers from and I am not prepared to have that suggestion driven around London on our buses," he said.
However, U.K. Christian groups Anglican Mainstream and Core Issues Trust, the groups behind the advert, accused the mayor of censorship.
Dr. Mike Davidson, Director of the Core Issues Trust, said: "We are saddened that this level of censorship is in operation in a city that regards itself as tolerant.
"We think that this is a mistake and we think it is very important that people have the opportunity to debate freely from all points of view."
Anglican Mainstream and the Core Issues Trust organized the advert in response to gay activist group Stonewall's advertising campaign in support of redefining marriage to include same-sex couples.
"Stonewall placed adverts on 1,000 buses we don't think it's unreasonable for us to come up with something that represents a very different point of view," Davidson argued.
Anglican Mainstream and Core Issues Trust had intended the bus adverts as part of a wider campaign to affirm conventional marriage between one man and one woman, as providing the best environment for the needs of children.
Canon Dr. Chris Sugden, Executive Secretary of Anglican Mainstream, said: "The current political debate surrounding the redefinition of marriage ignores not just the cultural base of this institution that lies at the heart of our society, but seems entirely to have forgotten about children, prioritizing adult sexualities at their expense in an unprecedented way."
[Daniel Blake contributed to this article]