Some former leaders of the so-called ex-gay movement, that believes that change in sexual orientation can be brought about through faith, have come out in opposition to the controversial conversion therapy, acknowledging the "terrible" emotional and spiritual damage it can do.

They include Jeremy Marks, founder of Courage UK, which was an ex-gay ministry but is now a pro-gay evangelical movement re-christened Two:23 Network.

Other signatories include Michael Bussee, Brad Allen and Yvette Cantu Schneider, of Exodus International, the ex-gay organisation that ceased its activities in June last year and apologised to LGBT people for the harm it had done.

In their open letter, published on the website of the National Center for Lesbian Rights in the US, they explain that conversion therapy, also known as "reparative therapy", "ex-gay therapy," or "sexual orientation change efforts", professes to help lesbian, gay, bisexual and transgender people to change or overcome their sexual orientation or gender identity. They say that majority of those who practise this "therapy" often do so with little or no formal psychological training, operating instead from a strict religious perspective, believing homosexuality to be a sin.

"At one time, we were not only deeply involved in these 'ex-gay' programs, we were the founders, the leaders, and the promoters," they continue. Together they represent more than half a century of experience. They say that few can be more knowledgeable about the "ineffectiveness and harm" of conversion therapy.

"We know first-hand the terrible emotional and spiritual damage it can cause, especially for LGBT youth. We once believed that there was something morally wrong and psychologically 'broken' about being LGBT. We know better now. We once believed that sexual orientation or gender identity were somehow chosen or could be changed. We know better now. We once thought it was impossible to embrace our sexual orientation or sexual identity as an intrinsic, healthy part of who we are and who we were created to be. We know better now."

They explain that they were simply teaching what they had been taught, that their identity needed mending. "We grew up being told that being LGBT was disordered, sick, mentally ill, sinful, and displeasing to God. We grew up being told that loving, same-sex relationships were shallow, lust-driven, deceived, disordered, and impossible."

Condemning conversion therapy as "ineffective and harmful", the leaders say they are now aligning themselves with every major mainstream professional medical and mental health organisation in denouncing attempts to change sexual orientation or gender identity. "We beseech the church to accept, embrace, and affirm LGBT persons with full equality and inclusion."

Mr Marks told Christian Today that in 1965, at the age of 13 when he first began to be aware of feelings that he was attracted to the same sex, there was no doubt in his mind that this was a terrible thing. "As a young man growing into puberty, I felt a deep sickening paralysing fear over my sexuality. I definitely did NOT want to be gay. So for at least the next 40 years of my life, I made every conceivable effort not to be."

This meant being born again, Christian discipleship and the way of the cross – crucifying the flesh. Hence he founded Courage in 1988, with the full backing of his local church.

"It took us about nine years to realise that all our efforts weren't achieving anything other than disillusionment with the process and disappointment with the long term results. But to change our theology to accommodate same-sex partnerships was absolutely out of the question. It took a further four years to realise that to doggedly pursue the same path was doing far more harm than good. It was not until about 1999 that I finally realised we must have got something terribly wrong and needed to start accepting loving committed same-sex partnerships – the only healthy viable way forwards that was demonstrating good fruit."

When he published his conclusions in 2000 he was expelled from Exodus International, his membership of the Evangelical Alliance was withdrawn, and, he says, was welcomed nowhere among the churches that had been his community since 1973 when he gave his life to Christ.

"Since then, gradually, I have been vindicated. Today I am proud to stand united with many fellow gay Christians and ex-ex-gay Christian leaders who have walked that same tortuous journey towards discovering that God simply makes some of us differently."

Tracey Byrne, chief executive of the Lesbian and Gay Christian Movement, welcomed the letter. "While the letter claims it's not an apology, it's hard not to be struck by the humility of the writers and by their powerful witness to the heartbreaking reality of these discredited and damaging programmes," she said. But she said there could be no cause for complacency. "Our mainstream churches, whenever they deny the identities, relationships and ministries of gay and lesbian people, wherever they demand celibacy or tell us were not quite what God had in mind, are playing into exactly the same culture which made reparative therapy, 'ex-gay' ministries, seem respectable and sensible responses."

Rev Sally Hitchiner, founder of Diverse Church, a movement for young LGBT people, said: "We are happy to read that so many respected Christian leaders have realised that trying to change someone's sexual orientation is neither safe nor effective. Many young people in our movement experienced prayer and counselling to change their sexual orientation and have suffered serious consequences for their mental health and relationships with church and family members who encouraged this. When it hasn't worked it has left the young people feeling they have to give up on God or sometimes even life. Whatever we may believe about gay relationships, accepting our young people as they are and looking for how God might want to use this for the wider church enables them to live a hope filled life with Jesus at the centre."