Columbia University psychiatry professor Robert Spitzer led a taskforce that in 1973 removed homosexuality from the official list of mental disorders contained in the American Psychiatric Association's diagnostic manual. The association also agreed that studies had shown that treatment to change sexual orientation could be harmful.

But Dr Spitzer said he decided that the study was necessary after talking with protesters who objected to the association's own policy on such therapies, which also discourages their use.

Other doctors and gay rights groups have criticised Dr Spitzer, calling his research "deeply flawed" as many of his subjects were referred by religious groups that condemn homosexuality.

At the association's conference in New Orleans, Dr Spitzer presented his telephone survey of people who reported that they had changed their sexual orientation with therapy and were happy with the decision at least five years later.

"The subjects' self-reports of change appear to be, by and large, valid, rather than gross exaggerations, brainwashing or wishful thinking," Dr Spitzer said in a summary of his paper.

"We, therefore, conclude that some individuals who participate in a sexual reorientation therapy apparently make sustained changes in sexual orientation."

His colleagues are calling the study "totally inadequate".

"For 30 years, Bob Spitzer may have been considered a careful researcher. But with this study, he no longer is. It is far from good science," said Harvard University professor and long-time researcher into homosexuality, Dr Lawrence Hartmann.

At the same time Dr Spitzer presented his findings, two other scientists reported on their studies of larger groups of men who underwent therapy and said they were harmed by it.

"The science Spitzer is presenting would not pass any peer-review system," said Dr Jack Drescher, a New York psychiatrist and one of the panel presenters.

Dr Spitzer's study was based on 45-minute telephone interviews with 143 men and 57 women who had sought help to change their sexual orientation.

He and his research colleagues found that 66 per cent of the men and 44 per cent of the women had achieved "good heterosexual functioning".

"If somebody wants to change and it's not because they are just responding to pressure, it shouldn't be automatically assumed that it's irrational or giving in to society," Dr Spitzer said in an interview.

Gay rights groups said that most of the subjects in the research were recruited through organisations that condemn homosexuality, such as Exodus, a Christian ministry that describes itself on its website as "promoting the message of `Freedom from homosexuality through the power of Jesus Christ'."

"It's snake oil, it's not science," said David Elliot, National Gay and Lesbian Task Force communications director.

The study has not been published or submitted for professional review. Dr Spitzer conceded that the number of homosexuals who could successfully become heterosexual was likely to be "pretty low". He also admitted that subjects in the study were "unusually religious" and were not necessarily representative of most gays and lesbians in the US.