The Christian group behind a banned bus advert suggesting gay people could be "cured" is seeking legal advice, the law firm acting for them has said.
A campaign was due to run for two weeks with posters on the side of the capital's buses bearing the slogan: "Not gay! Post-gay, ex-gay and proud. Get over it!"
Mayor of London Boris Johnson, who chairs Transport for London (TfL), ordered the adverts to be pulled following a public outcry about the Core Issues Trust campaign, backed by Christian group Anglican Mainstream.
Mr Johnson, standing for re-election next month, said: "It is clearly offensive to suggest being gay is an illness someone recovers from and I am not prepared to have that suggestion driven around London on our buses."
The ad responded to a recent campaign by Stonewall, which ran adverts saying "Some people are gay. Get over it" and claimed therapy could change sexual orientation. It was due to run on London buses from Monday.
Tom Ellis, a solicitor at law firm Aughton Ainsworth, said he had been contacted by Anglican Mainstream with a view to taking legal action against TfL, as the advert had already been cleared by the Committee of Advertising Practice before Mr Johnson stepped in.
He is now seeking an explanation from TfL of their justification for banning the ad and added: "It seems there is a decision that has been made that can be challenged."
Mike Davidson, director of the Core Issues Trust, has criticised the decision announced by Mr Johnson, saying: "I didn't realise censorship was in place. We went through the correct channels and we were encouraged by the bus company to go through their procedures. They okayed it and now it has been pulled."
TfL has said that the adverts are not "consistent with TfL's commitment to a tolerant and inclusive London".