Anti-gay adverts on London buses blocked by Boris Johnson - The Guardian

Boris Johnson, the Conservative mayor, has pulled an "offensive" Christian campaign advertising "gay conversion" which was due to appear on London's buses next week.

Revelations that adverts asserting the power of therapy to change the sexual orientation of gay people were due to be driven around the capital came as Johnson, who is seeking re-election in May, was due to appear at a mayoral hustings organised by the gay campaigning group Stonewall on Saturday.

The mayor immediately put the wheels in motion to halt the campaign after being alerted to the plans by the Guardian, and made clear that such advertising had no place in a tolerant city.

A clearly angered Johnson said: "London is one of the most tolerant cities in the world and 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."

Transport for London (TfL) subcontracts advertising for London's buses to the advertising agency CBSO, who ran the advert past the Advertising Standards Authority, according to one of Johnson's aides.

The ASA said the advert "does not infringe any advertising rules in the UK".

Johnson used his discretion as chair of TfL to instruct the transport body to ditch the advertising campaign which was due to appear on 25 buses driving around the capital next week.

The full-length advert was due to appear on five different routes in the capital over a two-week period. It is backed by the Core Issues Trust whose leader, Mike Davies, believes "homoerotic behaviour is sinful". His charity funds "reparative therapy" for gay Christians who believe that they have homosexual feelings but want to become straight. The campaign is also backed by Anglican Mainstream, an worldwide orthodox Anglican group whose supporters have equated homosexuality with alcoholism.

The advert was due to say: "Not gay! Ex-gay, post-gay and proud. Get over it!" Post-gay and ex-gay are terms used by some Christians and some psychotherapists and psychiatrists to refer to homosexual people who have undergone spiritual or pastoral therapy and, according to an Anglican Mainstream definition, have "now left a homosexual lifestyle [and experienced] an increased emotional and sexual attraction to the opposite biological gender and possibly a reduction in or loss of same-sex attraction."

Brian Paddick, the Liberal Democrat mayoral candidate who is gay, said: "From personal experience as a gay Christian, I can tell you that it's much better to be out than in. We should be celebrating the diversity for which London is known, not denegrating it.

"As mayor I want to make London a place that is welcoming to all people, including Christians."