In Britain today therapists are trying to convert gay men and women to heterosexuality. I know this, because for several months I infiltrated this network of therapists and put myself – a happy, "out" gay man – through treatment.

According to a report by Professor Michael King of University College London, one in six UK psychiatrists and psychotherapists have sought to reduce or change a patient's sexual orientation. And with the help of the American conversion therapy movement, practitioners here, along with a clutch of international "conversion" organisations, are becoming co-ordinated and unified. They plan to gain credibility, university backing and government funding. In some cases, the NHS is even paying for the treatment.

This is despite the fact that homosexuality was removed from the Diagnostic and Statistical Manual – psychiatry's glossary of conditions – 36 years ago. And despite much evidence that such practices are damaging and ineffective.

My investigation began last spring, shortly after King's report was published, when an evangelical group held a conference in a central London church for therapists wanting to learn how to "reorient" their patients. I wanted to know who these therapists were, what happened during the treatment, and what effect it would have on the recipient. I posed as a potential client wanting to be cured.

***

Two hulking security guards search me on my way in. Inside, there are two large lecture halls with more than 100 delegates. They are mostly men; they seem subdued, waiting for the show to start. Tables at the back of the hall display books on how to make people heterosexual.

The keynote speaker is Dr Joseph Nicolosi, an American psychologist and the author of some of the movement's core texts. He is the founder of the National Association for Research and Therapy of Homosexuality (NARTH), one of the biggest international conversion organisations. He has a cropped beard and wears a crisp suit.

"Homosexual behaviour is always prompted by loneliness," he tells the rapt audience with big gestures and a dramatic voice, "It's a pathology, a struggle to connect with the male identity."

His thesis is faintly Freudian: a distant father and an overbearing mother create deep wounds in a child, which lead to homosexuality. He speaks about the work at his own Californian conversion clinic. "We advise fathers, 'If you don't hug your sons, some other man will.' We train the mothers to back off.'"

During the lunch break gay protesters gather outside the venue, kept back by a police cordon. I can hear the din of the chanting and the whistle-blowing. The organisers advise us to stay inside.

I approach a psychiatrist, David, who had earlier asked a question from the floor, to see if he will treat me for my homosexuality. David tells me he runs a clinic which helps men "reach their heterosexual potential". He won't treat lesbians. "I have resolved my own sexuality," he says, explaining that he is now married with children, and gives me his business card: it reads, "I took the road less travelled." David points out a female psychotherapist who also practices conversion therapy, so I go over and introduce myself – I call myself Matthew.

She looks homely and her hair is greying. Her name is Lynne. She too gives me her business card. She is a fully accredited member of the British Association of Counsellors and Psychotherapists (BACP), the largest of the psychotherapy bodies.

After the conference I look David up online. As I'm researching his practice and qualifications, I see a reference to Iris Robinson, the scandal-stricken Ulster MP who in 2008 famously compared homosexuality to child abuse. In an interview with the BBC, she mentioned she knew a "lovely psychiatrist" who "tries to help homosexuals to run away from what they are engaged in." For the next step in my "treatment", I contact David and Lynne to arrange one-on-one sessions.

***

I have a dictaphone taped to my stomach as I arrive at Lynne's large house, north of London. She had told me beforehand that she would charge me £40 per session and that she always prayed at the beginning and end of the sessions. I'm shown into a spacious living room.

"I love my work and in particular this whole area of SSA [same sex attraction]," she says, as we sit down. "It's such an important area to work in." She has a wholesome face and the suburban air of someone who, when not trying to convert you to heterosexuality, would probably be rustling up a jolly good Victoria sponge. Like those at the conference, she doesn't say "gay"; she only uses the term "SSA".

I ask how she views homosexuality – as a mental illness, an addiction or an anti-religious phenomenon?

"It's all of that," she replies.

And then we pray. "Oh Father, we give you permission to work in Matthew's life to bring complete light and healing into every part of his being." After asking God to heal me, she opens her eyes. "I know the boundaries to keep within," she says.

She begins by asking me about my psychological history. I tell her that I was depressed as a teenager because I feared I would face prejudice for the rest of my life.

Can I learn to not feel attraction to men?

"Yes," she replies, "because that attraction is connected to a deep need that needs to be met and responded to and healed."

But how do I instead become attracted to women? Lynne explains that it's about "reprogramming" and going back into my early developmental stages. "Parts of you have developed but there is a little part of you that has stayed stuck," she says.

Oh, like being retarded?

"It is a bit like that," she agrees.

Lynne asks why I have come for treatment. I tell her that I'm tired of meaningless sexual encounters and that I have rediscovered my faith. She gets a whiteboard out and starts writing my words up on it. "I can't deal with the meaningless anymore," she says as she scrawls. "Hmm. Good sentiment."

"Are you feeling quite lustful with the SSA?" she asks. I reply that I am – but not just lust. In my last relationship, I say, I felt profound love towards my boyfriend. "That needs to be broken," she says. "There's a darkness that's very real that keeps you as its dog, but of course our God is more powerful than that."

Lynne's approach is two-pronged. She gives practical advice to intercept my sexual feelings towards men and, in keeping with Nicolosi's theories, delves into my past to search for my "wounds". These, she says, will explain why I turned to homosexuality.

She begins her wound hunt by asking about my family. I tell her that I have a close relationship with my parents and that they always gave me huge amounts of love, so I didn't understand why Nicolosi says that homosexuality is caused by inadequate parenting. "Well, there was something happening within your family dynamics that led to your depression," she says.

Lynne explains that people only identify as gay when they are already depressed. "There's a confusion, there's an anxiety, there's a lot of pain," she says. "Often the thought can be, 'Oh I'm confused about my sexuality so I must be gay'." She says that at the heart of homosexuality is a "deep isolation", which is, she says, "where God needs to be".

"Did you have a difficult birth?" she asks. No, I say. Why?

"It's just something I have noticed. Often [with homosexuality] it is quite traumatic, the baby was put into intensive care and because of the separation from the mother there can be that lack of attachment."

She moves on. "Any Freemasonry in the family?" No, I say, again asking her to elaborate. "Because that often encourages it as well. It has a spiritual effect on males and it often comes out as SSA."

Next, she looks for self-esteem wounds. "I think you have some unhelpful thoughts about yourself, about who you are," she says. "What do you think about yourself? In the deepest part of you, in your stomach."

"I think I'm a good person," I reply. She wants more. "I think I am a determined person." Still not enough. "I think I've a lot to give."

"But do you like yourself?" she asks, becoming impatient.

"I think I'm a good person," I repeat.

"Yes that's different though from 'do you like yourself?' Deep underneath this there's other stuff we need to get to. I think you must have had quite a lot of bullying." No, I say. "There was no sexual abuse?" she asks, leaning in and squinting again. No, I repeat. "I think it will be there," she replies, dropping her voice to a concerned tone. "It does need to come to the surface."

And so, she prays for me again. "Father, we give you permission to bring to the surface some of the things that have happened over the years. Father, enable your love to pour into that place of isolation in that little boy, whatever age, we give you permission to go there, with your healing power and your light, go into those parts, open all the doors, and access each one with your light."

She looks up. I ask her again about this abuse. "I think there is something there," she says. "You've allowed things to be done to you." In the next session I ask if she thinks the abuse would have taken place within my family, because I can't remember it. "Yes, very likely," she replies.

The following session takes place on the phone as Lynne is abroad. This time, she focuses on the practical. She recommends that I distance myself from my gay friends and take up a sport such as rugby.

I ask what I should do about masturbation – is it best to abstain completely? "It is, it definitely is," she says. "It will be a battle, but the more you can say no to it, the stronger you get. The enemy is going to bombard you."

She tells me what to say internally when I think about an attractive man: "Father I need help, I know it's wrong, you have all the power over my thoughts and I give that image to you Father, and I ask that you will help me to put the right image in my mind."

Lynne recommends I read a book called Setting Love in Order by Mario Bergner, an "ex-gay". In it, he claims that through prayer he also managed to cure himself of HIV. So with prayer can an HIV-positive person really become negative? "Well the Lord heals, doesn't he?" she replies. "Are you HIV-positive?"

It transpires during the sessions that she gets most of her clients through an NHS GPs' surgery near her home. She says they refer people to her for treatment for their homosexuality. I tell her I know someone in that area that wants referring, should he just say its for anxiety and depression? "I think it would be better to say anxiety and depression initially and then we can take it from there," she replies. "He can usually get four sessions with the practice, which are paid for by the NHS."

At the end of the session I ask about my chances of success with this treatment. "It's down to what you want," she says. "Good will come out of this. The Lord will lead us through."

***

I am being cured of my homosexuality via Skype. We use webcams throughout to see each other. David holds my gaze constantly. There are more psychotherapeutic models and theories and a little bit less God, but the agenda is the same: you have been wounded.

Like Lynne, David takes a psychological history. He asks me why I'm seeking treatment and says that, with a bit more investigation, he can tell me why I became gay.

His prognosis is optimistic. "One third of people change completely," he says, "one third of people experience significant change, and one third don't experience change. Those people may have been more deeply wounded." He says you have to find the wounding in order to move forward.

So he sets about finding mine. But this isn't his only tool – his central thesis is that you have to replace homosexual sex with what he calls, "the Four Intimacies: intimacy with God, intimacy with oneself, intimacy with the masculine and intimacy with the feminine." By strengthening your relationships in all these areas, and particularly by having more platonic contact with men, he says, your need for sexual contact is negated.

David sets out an action plan. He recommends I join Christian men's groups. "Often there are [for homosexuals] a lot of wounds around masculine community," he explains. He also gives me "exercises" to do. These include standing in front of the mirror naked, touching and "affirming" myself. He makes another such suggestion. "A man may choose to go for a massage as a way of having healthy contact [with another man]," he says.

In the next session I tell him I have followed his advice. "It made me aroused," I say about the massage.

"An erection is just an erection," he replies. "All that indicates is that your body has been programmed that [sex] is the only way men have physical intimacy."

But how do I interrupt lustful thoughts?

"The attraction isn't the problem," he says. "It's the story that you tell yourself of what the attraction means. Ask yourself why you find a man attractive. Is it because he's got broad shoulders? Well, what do the broad shoulders mean to you?"

He then asks if I know the work of someone called Elizabeth Moberly. "She talks about the cannibalistic nature of homosexual sex," he says. Cannibalistic? I ask. "If I see you're a younger man," he explains, "and the story I tell myself is that younger men are fitter, or more powerful and I'm feeling particularly weak, then suddenly you have something that I want to possess."

"What kind of men do you find attractive?" he asks.

"I don't have a type," I say. He looks displeased. "Sometimes when I was younger I went for older men," I add. He asks me about my childhood. I tell him it was happy. "Any big traumas?" he asks. No, I say. "Any sexual or physical abuse?" Again, he finds nothing.

"Tell me about your father." I say that he was great, supportive and that we are very different. He is scientific and introverted, whereas I am more like my mother: creative, extrovert. This is a breakthrough.

"So in your mind there's something that says, 'I'm like mum, but dad's over there, he's different from me,' so there hasn't been that gender-affirming process. When puberty kicks in, those natural needs for masculinity become sexualised. Suddenly older men want to have sex with you, and it's pretty intoxicating. That's what's lead you down the line of homosexuality.

"But the men you were having sex with or falling in love with are just as wounded as you," he adds. "They cannot complete you in the same way as a woman. What would complete you is sex with a heterosexual man but a heterosexual man isn't going to want to have sex with you, so it's that desire to get what you can never obtain."

So is homosexuality a pathology? "It does represent a pathology," he replies. "Often the dynamics behind it aren't healthy. God's intention for us is to have an opposite-sex relationship."

I suddenly remember his wife and children. "You're very much a success story then," I say.

"Well, I mean it works," he replies. "I never went down the line of gay identifying, it was something I experienced that I had disgust around and I always wanted to be married and have kids."

What about his same sex feelings now? "Sometimes if I'm not looking after myself then that can bring up a sexual charge," he replies, "but it's not a big issue for me now, it's more unhealthy patterns of porn and masturbation, if I'm feeling a bit flat."

In the following session I tell David I've had sexual thoughts about him. "Thank-you for being honest," he replies with a small laugh. "I'm trying to model unconditional love for you, so it is very natural that you would have sexual thoughts related to that. Although we're doing this over the internet, there is still a potential for a sexual connection. So there's probably a part of your mind that's thinking that through."

"Do you have to deal with sexual feelings towards men you're working with?" I ask.

"I get echoes of it sometimes," he replies. "I'd be lying if I said I didn't."

I tell him that I had tried the standing-in-front-of-the-mirror-naked technique that he recommended, but, like the massage, it had aroused me. "I would be surprised if you didn't experience sexual feelings," he says. And with that he starts to "affirm" me.

"I think you're a brave man," he says. "I think you're really strong in terms of being willing to look at your life and who you really are, and you also look as if you look after yourself in terms of your body. How do you feel being affirmed in this way by another man?"

I say that when men compliment me on my appearance it triggers sexual feelings. He probes again, asking me how I'm feeling as he talks about my body. Aroused, I repeat. But rather than moving away from this apparent sexual trigger, he asks if we can do an "exercise" around it. I agree.

"Close your eyes and focus on that arousal you're feeling down in your genitals," he says. "I want you to hear, as a man, as I look at your body, I see strong shoulders and a strong chest, I see a man who has an attractive body and I want you just to notice the arousal you feel as you hear me talking about that. Imagine an energy and picture that energy as a colour, and make the brightness of the colour relate to the intensity of the sexual feeling, so you might be starting to get a bit of a hard on, you might be starting to feel an erection and that sexual energy, but I want you to just picture that as a coloured light. What colour would it be?"

Red, I say.

"I want you to imagine that red colour, that energy and listen to the affirmations that I see you as a strong, confident man, and I want you to move that red light from your genitals up into your chest to join that feeling of affirmation as a man, and as you breathe in that affirmation do you notice now what happens to the arousal?"

I tell him it's still there. We're at the end of the session. I ask who his supervisor is. He tells me that his supervisor is involved with "Richard Cohen's organisation".

Later, I do an internet search. Richard Cohen has been permanently expelled from the American Counseling Association. He founded the International Healing Foundation, a conversion organisation.

David then mentions that he is attending a meeting in London next week to discuss how to create a training school for therapists working in this field. I ask if I can come too.

A week later I am sitting round a table with the heads of every major British conversion organisation – some of whom are also therapists. We are in the bright, cramped London offices of Living Waters, an evangelical conversion association. Our special guest is Dr Jeffrey Satinover, 62, an American psychiatrist.

"We need to think of practical ideas for forming a training programme [for conversion therapists]," says Satinover. "We can learn a lot from what the Mormons did. They created a training programme and linked it with a university."

They decide that in order to gain such a link, and the credibility and funding that would go with it, their training school also needs to do research into the field.

"If I could get trainees doing a PhD in this area that would help, because that brings funding in," says David. "It's difficult to access money, but if we're clever there are ways." He talks about his conversion practice, and how it's funded. "We use people's private medical insurance," he says.

***

I phone Lynne. I explain that I'm a journalist. "Who is this please?" she says, confused, or in denial. I repeat myself and then read to her what the psychiatrist Professor Michael King from UCL had to say about her practices when I contacted him with transcripts of the sessions:

"This is grossly improper practice," he told me. "She's imposing prayer and using evidence-free techniques. The whole approach towards the subject of sexual abuse is extremely unprofessional. Leading [and] suggestion in a therapeutic situation is the absolute antithesis of what an exploration of sexual abuse should be about. It's the base of many of these false memory syndromes. She should not be able to get referrals from a GP. Her membership of the BACP should be immediately revoked."

Lynne is silent. Finally, she summons her defence. "When I take someone on, I work with the person's value system," she says. "You told me you were a Christian so that then means that I work in that person's value system. Everything I did was totally within the BACP's ethical framework."

I quote one of their guidelines: "Practitioners should not allow their professional relationships with clients to be prejudiced by any personal views they may hold about sexual orientation."

She hangs up. I phone the GP surgery and get through to one of the partners. Is it true his practice refers patients to Lynne to treat their homosexuality?

"That is not true," he says, before admitting that he has never had a discussion with the other partners in the practice about this. "If the patient said that's what they wanted we can't stop them going for help," he says.

Later I lodge a complaint with the BACP about Lynne.

David remains completely calm when I reveal I am a journalist. As with Lynne, I read out what King said about his practises:

"None of this would be recognised within psychiatric practice. No psychiatrist could ever justify explaining their fantasy life to a patient. It's not good practice to talk about your own psychotherapy or your own difficulties. He's encouraging an arousal during a session – it's like a hypnotic technique. I wonder what he's doing with patients – that he is at risk of crossing the line. My personal opinion is that he is going against every code of practice from the RCP. The College should withdraw his membership."

"All I can say is that I try my best to deliver a service to help people, to provide people with an alternative in terms of change," David replies. "It [his treatment] is only for people who come asking."

I lodge a complaint about David with the GMC.

The purpose of this investigation was to find out how conversion therapists operate. What I didn't expect was that I would learn how their patients feel: confused and damaged.

I began to constantly analyse why I found particular men attractive. Does that man represent something that's lacking in me? Do I want him because he looks strong which must mean I feel weak? Did something happen in my childhood? The therapists planted doubt and worry where there was none.

My experiences, I learn, are typical. I speak to Daniel Gonzalez, one of Nicolosi's former clients. "Conversion therapy is a very complicated form of repression," he says. "It's a way of convincing yourself that your same sex attractions have some alternate meaning. It continued to haunt me for years."

I also speak to Peterson Toscano, who spent 17 years in Britain and the US trying every different reorientation treatment available. He says simply: "It's psychological torture."