The sample is terrible, totally tainted, totally unrepresentative of the gay and lesbian community

David Elliot, US National Gay and Lesbian Task Force

He said he cannot estimate what percentage of highly motivated gay people can change their sexual orientation.

But he said the research "shows some people can change from gay to straight, and we ought to acknowledge that."

Dr Spitzer conducted 45-minute telephone interviews with 200 people, 143 of them men, who claimed they had changed their sexual orientation from gay to heterosexual.

They answered about 60 questions about their sexual feeling and behaviour before and after their efforts to change.

Most said they had used more than one strategy to help them change.

About half said the most helpful method was to work with a mental health professional, mostly commonly a psychologist. Others used books, or mentoring by a heterosexual.

Dr Spitzer concluded that 66% of the men and 44% of the women had arrived at what he describes as "good heterosexual functioning".

He defined this term to mean being in a sustained, loving and sexually active heterosexual relationship within the past year.

No convincing evidence

Psychologist Dr Douglas Haldeman, of the University of Washington, said the study offered no convincing evidence that people's sexual orientation had been changed.

He also said the participants appeared unusually skewed toward religious conservatives and people treated by therapists "with a strong anti-gay bias."

Dr Haldeman said such participants might think that being a homosexual is bad and feel pressured to claim they were no longer gay.

He added that some 43% of the sample had been referred to Spitzer by "ex-gay ministries" that offer programmes to gay people who seek to change.

An additional 23% were referred by the National Association for Research and Therapy of homosexuality, which says most of its members consider homosexuality a developmental disorder.

David Elliot, a spokesman for the National Gay and Lesbian Task Force in Washington, said: "The sample is terrible, totally tainted, totally unrepresentative of the gay and lesbian community."