The ad mimics the wording of the 'Gay. Get Over it!' campaign by rights group Stonewall, but states, "Not Gay! Ex-Gay, post-gay and proud. Get Over it!". It had been booked for a two-week campaign starting next Monday (16 April).
The phrases "ex-gay" and "post-gay" are used by Evangelical Christians to describe people who they believe have been ‘cured’ of homosexuality through the power of prayer and Christian therapy.
The ad campaign was booked by Core Issues, a Christian charity that says it provides "therapeutic support" for people with "homosexual issues", and Anglican Mainstream, a network committed to promoting the traditional biblical teaching on marriage, the family and human sexuality.
A post on the Anglican Mainstream website said both organisations "recognise the rights of individuals to identify as gay, and to live according to their own values," but individuals "should be supported in developing their heterosexual potential".
The ad campaign was banned by TfL yesterday. Boris Johnson, mayor of London, said: "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."
Stonewall's ad, which is currently running on 1,000 London buses, says on a similar red background, "some people are gay, get over it!" and links back to the charity's equal marriage campaign website.
Dr Mike Davidson, co-director of the Core Issues Trust, said: "We are very concerned that Boris Johnson has been exercised in this way. He is acting like a 13th Century bishop, speaking the word and everyone has to respect what he says.
"It is not our intention to disrespect gay people who are happy but we are responding to Stonewall's ad and it is important that all sides of the argument are heard. There are people who want to move away from homosexuality."
A TfL spokesperson said: "This advertisement has just been brought to our attention by our advertising agency, CBSO and we have decided that it should not run on London's bus or transport networks.
We do not believe that these specific ads are consistent with TfL's commitment to a tolerant and inclusive London. The adverts are not currently running on any London buses and they will not do so."
It is not the first time bus ad campaigns have caused controversy. The Outdoor Media Centre (then known as the Outdoor Advertising Association) was forced to axe an ad by Beta. The ad promoted the power of outdoor and was widely criticised in January 2012 for its "career women make bad mothers" slogan.
In January 2009 the British Humanist Association ran an ad campaign that said: "There's probably no god. Now stop worrying and enjoy your life."
It was created in response to a series of Christian bus adverts and received more than 100 complaints.
Follow Maisie McCabe on Twitter @MaisieMcCabe
This article was first published on campaignlive.co.uk