They described her as "reckless", "disrespectful", "dogmatic" and "unprofessional". They said she showed "no empathy" towards her client. Why? Psychotherapist Lesley Pilkington had tried to turn a gay person straight.

In a landmark ruling this week, Pilkington, 60, was found guilty of "treating" a patient for his homosexuality. A hearing of the British Association for Counselling and Psychotherapy – the largest professional body for therapists – concluded that the treatment she gave constituted "professional malpractice".

The unanimous verdict came with heavy sanctions. Pilkington's accreditation to the organisation was suspended. She was ordered to complete extensive training and professional development. If she does not file a report in six to 12 months, satisfying the board that she has complied, she will have her membership fully revoked: she will be struck off.

The report concluded: "Mrs Pilkington had allowed her personal preconceived views about gay lifestyle and sexual orientation to affect her professional relationship in a way that was prejudicial."

The client Pilkington tried to cure was me. I am an out, happily gay man. I was undercover, investigating therapists who practise this so-called conversion therapy (also known as reparative therapy) – who try to "pray away the gay". I asked her to make me straight. Her attempts to do so flout the advice of every major mental-health body in Britain.

But despite the decades of abuse that gay patients have received from therapists and psychiatrists – despite the electro-convulsive therapy used until the 1980s, despite the chemical castrations, the aversion therapy (where pain is inflicted to dissuade same-sex fantasies) and despite the recent rise in fundamentalist talking therapy – no one has ever been held to account.

The details of this case, and another I am pursuing, explain why not only gay clients but mental-health patients in general do not come forward to complain. Psychiatrists and psychotherapists routinely avoid accountability – and the government is helping them do so.

My investigation began in April 2009. I heard that a conference was taking place in London for therapists and psychiatrists who wanted to learn how to convert their patients to heterosexuality. Homosexuality was removed from psychiatry's glossary of mental illnesses in 1973. How then could anyone treat something healthy? I went along to find out, posing as someone looking to be "cured". Two people agreed to treat me. The first was a psychiatrist – we'll come to him later. The second was Lesley Pilkington.

A few weeks later I was in her grand Hertfordshire home with a Dictaphone taped to my stomach. She set about trying to find the childhood "wounds" that she believes led to my homosexuality. But she found none. "There was no sexual abuse?" she pressed.

"No."

"I think there is something there . . . you've allowed things to be done to you." She then prayed: "Father, we give you permission to bring to the surface some of the things that have happened over the years." I asked who could have committed this abuse – a member of my family? "Yes, very likely," she replied.

Was homosexuality a mental illness, an addiction or an anti-religious phenomenon? "It's all of that," said Pilkington. During the sessions, she recited prayers for me to say whenever I thought about a man sexually. She gave me how-to-be-heterosexual tips such as taking up rugby, abstaining from masturbation and distancing myself from gay friends.

When the results of my investigation were published last year in the Independent, it sparked widespread outrage. Not least because Pilkington claimed that she had had referrals to "treat" gay clients from the NHS GP surgery to which she is attached. As a result of the investigation, the British Medical Association passed a motion condemning conversion therapy and calling on the NHS to investigate instances where it may have unwittingly paid for it.

Just before its publication, in January 2010, I made a formal complaint about Pilkington to the BACP. But by last autumn, little had happened. Three dates for a hearing were made and then cancelled. The BACP, which has 32,000 members, explained that they couldn't find people for the adjudication panel. Why? "The legal advice we've been given is that the panel members can't be very religious but nor can they be overtly pro-gay," said Fay Reaney from the professional conduct department. So in a complaint about racism would they therefore not allow someone on the panel who is strongly opposed to racism? "This is the advice we've been given," she replied.

A new date – 20 January – was confirmed. Four days before the hearing Pilkington gave an interview to the Sunday Telegraph, contrary to BACP guidelines that neither party speak publicly about the case. I had not named her in my original article. She then went on the radio to talk about it. In response to Pilkington's disclosures – 48 hours before the hearing was due to take place – the BACP adjourned it and issued us both with confidentiality agreements.

The signed agreements would have prevented either side from ever talking about the case. My barrister, Sarah Bourke, advised me not to sign. But I couldn't decide. I didn't want to jeopardise the case but was it worth pursuing if it could never be discussed publicly? The BACP wouldn't tell me what would happen if I refused to sign.

Meanwhile, Pilkington's representatives – the Christian Legal Centre – were making intriguing claims. On the day the hearing would have taken place, they stated that it had been postponed because one of the expert witnesses she had cited in her defence had been subject to "menacing phone calls, threats and intimidation". I was the only person named in her lawyers' statement. Although she submitted testimony from several witnesses, I never knew their names and the BACP did not call any of them.

But the Daily Mail ran a story regardless: "Trial of therapist who tried to 'cure' gay man is halted after 'expert defence witness is intimidated'," screamed the headline. Countless Christian websites repeated the claims. Hate mail poured in. Pilkington continued to give interviews and gave a talk at another conversion-therapy conference in London. With the agreements unsigned, the BACP decided to go ahead regardless. What was the point of adjourning the case for four months? The BACP would not explain.

Finally, the date was set. During the hearing, Pilkington said she still "feels there's a need" for my homosexuality to be treated. The panel asked her if it was good practice to say to someone who had stated they had not been sexually abused: "You've let things be done to you." She replied: "It didn't come across like that."

Was it, the panel asked, her belief that homosexuality was wrong, sinful or unnatural? "Oh yes," she replied. "There's no question about that . . . but there's a way out."

Pilkington revealed that she was trying to convert another gay client to heterosexuality. But that now she's "clearer" about it – she uses a contract adapted from a US-based conversion-therapy organisation. Equally startling, however, was what the panel asked me: on what basis did I assert that the BACP was publicly opposed to conversion therapy? I read aloud the letter the BACP had written to the Guardian in 2009 describing such therapy as "absurd" and stating that it "makes people with gay thoughts suffer extra pain". The panel was unaware of the letter and the BACP's position on the subject. After lunch the chair announced that they would disregard the statement as they "don't know who authorised it".

As the hearing progressed, I discovered the strain all complainants go through. I was cross-examined at length by Pilkington's barrister and by the panel. How would someone with mental-health problems cope with that? And it isn't just the emotional challenges that could deter a complainant. Without being well educated and having free legal help to interpret the BACP's jargon-dense literature and legal letters, I would have found the process incomprehensible and intimidating.

The BACP's ruling in the Pilkington case will, however, help to reassure the victims of conversion therapy. Since my first article was published dozens of people have contacted me describing their experiences. Young people whose parents had forced them into residential gay "cure" centres in the US deep south. Middle-aged men and women who wasted decades trying to be straight. Several people who had attempted suicide. One young man showed me the self-harm scars on his arms. I thought about him every day.

But although this case will serve as a precedent, it does not solve the wider problem. Even if Pilkington had been struck off completely she would still be able to carry on practising. Anyone can claim to be a therapist in Britain because there is no state regulation of the profession. "Psychotherapist" and "counsellor" are not protected titles. The BACP is a self-regulating, independent body. No one has to be a member. Thus you can't stop a bad therapist seeing clients any more than you can a fortune-teller.

The previous government had planned to regulate counsellors and psychotherapists by bringing them under the Health Professions Council, in line with other health workers, such as chiropodists, hearing aid dispensers and art therapists. This would have provided a central body offering standardised codes of conduct. But, contrary to the advice of mental-health charities such as Mind, the coalition has decided not to do this. Instead, the HPC will introduce a voluntary register for therapists.

But there is another unsettling thread to this story: that of the psychiatrist. His name is Dr Paul Miller. After meeting him at the London conference, he agreed to "treat" me for my homosexuality via Skype – as he lives in Belfast. He claims to have "resolved" his own conflicted sexuality and is now married with children.

Miller told me that homosexuality "represents a pathology". He added: "The men you were having sex with or falling in love with are just as wounded as you." He concluded that because my father is a physicist, and I was always more creative, that prevented a "gender-affirming process" which in turn led to my sexualising men.

His advice was for me to have massages with male masseurs and to stand in front of the mirror naked, touching myself, thus somehow affirming my masculinity/heterosexuality. He told me to visualise a red light when aroused: "I want you to move that red from your genitals up into your chest," he said.

I complained to the General Medical Council (the Royal College of Psychiatrists has no remit for disciplinary procedures). The RCPsych has stated: "There is no sound scientific evidence that sexual orientation can be changed." Yet the GMC let Miller off without even a warning – in fact, without even a hearing.

After receiving my complaint they appointed a consultant psychiatrist – whose identity was redacted – to write a report about the taped evidence I submitted. The crux of the report was that conventional therapeutic practices used by many psychotherapists have "as much or little scientific evidence" as conversion/reparative therapy. And yet reparative therapy is based on the work of self-proclaimed psychologist Elizabeth Moberly, who is not trained – her degree was in theology – and whose theories were not based on clinical research. The professional guideline document Good Psychiatric Practice, to which all psychiatrists are bound, states: "A psychiatrist must provide care that does not discriminate and is sensitive to issues of sexual orientation." The GMC report relating to my experience concludes: "I do not consider that Dr Miller's actions were inconsistent with Good Psychiatric Practice." I will appeal.

Reaction to the report has been unrestrained. The psychiatrist and author Dr Max Pemberton told me: "The GMC's decision is scandalous. Conversion therapy has been shown consistently to be dangerous and damaging. It is a disgrace that a qualified doctor is engaging in such practice, and an even greater disgrace that the GMC do not appear to feel that this warrants their attention."

A 2002 study by US clinical psychologists Ariel Shidlo and Michael Shroeder found that 55% of patients experienced psychological harm from conversion therapy, the results of which included depression and suicide attempts.

Furthermore, as Michael King, professor of psychiatry at UCL, points out: "There is an error in the GMC's logic: homosexuality is not a diagnosis. To therefore offer any kind of treatment can be damaging." He added: "Self-regulation is a problem. Professions are inward looking. People don't like to criticise each other."

But until the government steps in, self-regulation will continue to protect psychiatrists and therapists. Dissatisfied patients, meanwhile, will be deterred from complaining.

Dr Miller is still practising in his clinic in Belfast. Lesley Pilkington can carry on charging patients and praying for God to "bring to the surface" their non-existent traumas. No one can stop them.