Psychotherapists are offering to help "cure" gays and lesbians of their homosexuality without any evidence that such treatment is beneficial or safe. One in six said they had tried to turn gays straight, or reduce their gay or lesbian feelings, even though the mainstream medical view is that this is impossible.

The idea that homosexuality can be cured has a long and dubious history and the disclosure that a significant minority of therapists and doctors still think it is possible is "worrying", Professor Michael King, of the University College Medical School, said. "Heaven knows what they do. We didn't attempt to ask them because there is no evidence that anything works. We didn't expect it to be happening at this rate and we are really rather concerned. It ought to stop. It is distressing and harmful and there is absolutely no evidence it works," he said.

A study of more than 1,400 psychiatrists and therapists in BMC Psychiatry found that 222 (17 per cent) said they had treated at least one client to alter their homosexual feelings at some point. The researchers expected the cases to be concentrated in the past, but the 400 to 500 cases recorded were distributed evenly across the decades. "It is happening up to the present moment," Professor King said. It might only be the "tip of the iceberg".

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Many therapists seemed uncomfortable with giving treatment, or admitting to it. When asked if they would attempt to change someone's sexual orientation if requested, only one in 25 (4 per cent) said they would – far fewer than the one in six who reported actually doing so. Pressure from clients demanding help because of bullying or discrimination or family pressures may have pushed the therapists into delivering it, the professor said.

The idea that homosexuality is an aberration from the norm which can be "corrected", rather than a natural state, was current for most of the last century. Everyone was thought to be basically heterosexual and homosexuality was regarded as a deviation from the norm, the result of "faulty learning" in childhood.

During the 1950s and 1960s, when belief in psychological behaviourism was at its height, aversion therapy was used to "cure" homosexuals. Male patients were given a slide show which included pictures of sexually attractive men and women and a lever that allowed them to change the slides. If they lingered too long over the pictures of the men, and did not move on swiftly enough to the pictures of the women, they received an electric shock. A variation of this treatment involved a drug that would make them vomit.

Aversion therapy, famously employed in Anthony Burgess's novel A Clockwork Orange to cure Alex, the leader of the Droogs gang, of his obsession with violence, was used up to the 1980s, but has since been discredited.

Other treatments included advice to masturbate to a homosexual fantasy and then switch to a heterosexual one near orgasm. Covert sensitisation was a method which required patients to counter homosexual thoughts with shameful fantasies of arrest by the police or discovery by their family. Hypnotherapy and psychoanalysis were also used.

Although not uncommon, these treatments never became mainstream in Britain. In the US, however, the idea that homosexuality can be cured retains wide support. There is an ex-gay movement, led by right-wing Christian groups, which promotes "reparative" therapy to return people to the heterosexual "norm". Supporters cite findings that depression is two to three times more common among young gay men as evidence that a homosexual orientation is itself harmful – rather than the responses of bullying and discrimination that it engenders.

Professor King, head of the psycho-sexual clinic at University College Hospital in London, said: "We do not have the same attitudes in Europe. But young people go on the internet, they see this stuff and they pop off to see a psychotherapist. If the therapist is not wise enough to say that this is a part of them and there is nothing pathological about it, they may get seduced into trying to change them. Instead, the therapist should be saying that it is very unfortunate they are being bullied and that they can try to help them come to terms with their situation and learn to cope with it."

Derek Munn, of the gay rights organisation Stonewall, said: "So-called gay cure therapies are wholly discredited. The conclusions of this research are a welcome reminder that what gay and lesbian people need is equal treatment by society, not misguided treatment by a minority of health professionals."

A patient's story: 'I got an electric shock if I looked at the man'

I think I always knew I was gay but there was, in the Sixties, an enormous social pressure on you not to be. I was a day pupil at a boarding school and there was some fairly broad-minded sexual play. Some people stayed there and some moved on to heterosexual activity – while your peer group and natural development is telling you that there is a very fuzzy edge, society is telling you that there is this very hard, black-and-white precipice. So, it becomes an area that you don't talk about with parents and people like that.

While I was a student back then, I had some successful heterosexual relationships. However, during that time I got involved in a strong relationship with a school friend which went on for a long time. When that broke up, it caused me a lot of anguish.

During my early twenties (the early 1970s), I became increasingly depressed and went to my GP. I ended up meeting one of the leading lights in treatments for homosexuality at the (local) university department of psychology. He used aversion therapy and an electric shock machine that was tied to the ankles and wrists. You then watched a number of slides, some [of men] which you find sexually attractive, some which [of women] were best identified as your heterosexual goal.

If you switch from the "gay" picture to the "heterosexual" picture then you don't get the electric shock. By today's standards, the pictures used were about as stimulating as a Reader's Digest. Each session lasted 30 or 40 minutes and I had about 30 sessions. In the fullness of time I got married and the sex thing... well, it never worked out.

My wife knows about these gay feelings. She tends to regard them as a threat in her own mind. It's still a very, very sensitive area. A few casual but long-term sexual friendships ensued with people who were – almost without exception – married.

My wife knows of these encounters and has tolerated them with increasing difficulty.

Increasingly I feel betrayed by the promises of treatment. After I left treatment where was the back up? My deepest feelings, the very structure of my being, had been torn apart in the name of science and left abandoned while the psychologists got on with building their own careers and lives.