The research funded by the Wellcome Trust has been published in the open access journal BMC Psychiatry.
To reach the conclusion, researchers from UCL (University College London) and St George's, University of London, question over 1,400 mental health professionals on whether they would attempt to change a client's sexual orientation if requested. Although only one in twenty-five said that they would do so, one in six reported having assisted at least one client to reduce their gay or lesbian feelings, usually through therapy.
Therapists were also asked in what year they had conducted such therapy and there was no sign of a decrease in recent times.
"There is very little evidence to show that attempting to treat a person's homosexual feelings is effective and in fact it can actually be harmful," says Professor Michael King from UCL.
"So it is surprising that a significant minority of practitioners still offer this help to their clients," the expert added.
Professor King and colleagues found that a number of reasons were given by the psychiatrists and therapists for offering assistance, ranging from the counsellor's own moral and religious views about homosexuality through to a desire to help patients who were stressed by discrimination.
There was also a degree of ignorance about the lack of evidence surrounding such the efficacy of such therapies - in particular, that no randomised control trials have ever been conducted that show that the therapies are effective.
Professor King believes that it is important to raise awareness amongst both therapists and the wider public about homosexuality and its so-called treatments.
"The best approach is to help people adjust to their situation, to value them as people and show them that there is nothing whatever pathological about their sexual orientation. Both mental health practitioners and society at large must help them to confront prejudice in themselves and in others," he said.