London Mayor Boris Johnson yesterday blocked a bus advert suggesting that homosexuals could be cured.

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.

Although advertising regulators had cleared the advert, Mr Johnson used his position as chairman of Transport for London to ban it from the capital’s buses.

He said 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.

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 organised the advert in response to gay lobby group Stonewall’s advertising campaign in support of gay marriage.

“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,” Dr Davidson added.

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, prioritising adult sexualities at their expense in an unprecedented way.”