UK doctors have denounced plans to promote medical treatment for homosexuality at a conference organised by the church group Anglican Mainstream.
A prominent American psychologist Dr Joseph Nicolosi is to talk about how he had helped many people to become heterosexual.
The Royal College of Psychiatrists (RCP) retorts that there is no supporting evidence and warns such treatment could be damaging.
It noted the American Psychiatric Association had concluded there was no scientific evidence that homosexuality was a disorder and removed it from its diagnostic glossary of mental disorders in 1973.
The World Health Organisation's International Classification of Diseases followed suit in 1992.
Dr.Nicolosi claims he has been helping people to "increase their heterosexual potential" for 25 years, and put his success rate among men at about two out of three.
He told BBC News that he was offering a choice for people who were unhappy being gay.
While the Church of England said it did not promote such therapies, Anglican Mainstream believed such an approach needed to become more well known.
Dr Nicolosi said: "We have a great deal of evidence showing that these individuals are not harmed and that the therapy does work.
"We are petitioning the American Psychiatric Association to look at the scientific data."
An RCP spokesman said: "There is no sound scientific evidence that sexual orientation can be changed.
"Furthermore, so-called treatments of homosexuality create a setting in which prejudice and discrimination can flourish."