A Christian group has launched a High Court "battle for free speech" after its advert suggesting that gay people can be "cured" was banned from the sides of London buses.
The Mayor of London, who is in charge of Transport for London (TfL), rejected the ad posters as "offensive to gays".
Boris Johnson has also warned that displaying the ad could lead to a retaliation against the wider Christian community.
The rejected posters say: "Not Gay! Ex-Gay, Post-Gay and Proud. Get over it!"
They are in response to a poster campaign by gay rights group Stonewall, which plastered on the sides of the capital's red buses with the message: "Some people are gay. Get over it!"
The banned ad promotes the view of the Core Issues Trust that homosexuals can be "reoriented" through therapy and prayer.
Dr Mike Davidson, the head of the trust, asked Mrs Justice Lang to rule at London's High Court that the ban was unlawful and "a deep threat" to democratic freedoms.
Paul Diamond, appearing for Mr Davidson, told the judge: "We believe this is a very important free speech case on whether a totally temperate, restrained advertisement can be put on the sides of London buses."
Before the hearing started, Mr Davidson said: "I hope this hearing will give us the opportunity to put the case for securing some kind of future for people who want to move away from homosexuality and to be recognised as a valid group with needs to be protected and respected."
A TfL spokesman said: "The advertisement breached TfL's advertising policy as in our view it contained a publicly controversial message and was likely to cause widespread offence to members of the public."