One of the most influential psychiatrists of the 20th century has apologised to the gay community for a study supporting the use of therapy to ‘cure’ homosexuality.

The study, titled ‘Can Some Gay Men and Lesbians Change Their Sexual Orientation?’ was published in the Archives of Sexual Behaviour in 2003 and has long been hailed by American conservatives as proof that being gay is abnormal.

But author Dr. Robert Spitzer has now issued an apology, calling his original study “fatally flawed”.

The apology letter appears in the latest issue of the same academic journal that published his controversial research.

“I believe I owe the gay community an apology for my study making unproven claims of the efficacy of reparative therapy,” the letter reads.

“I also apologise to any gay person who wasted time and energy undergoing some form of reparative therapy because they believed that I had proven that reparative therapy works with some ‘highly motivated’ individuals.”

Spitzer’s study surveyed 200 men and women undertaking ‘gay conversion’ therapy, a large number of whom had been referred to him by religious groups. After analysing their sexual feelings and urges following the therapy, he concluded that “some people can change from gay to straight”.

In his apology letter, Spitzer acknowledges his study’s “fatal flaw” – namely, that “there was no way to judge the credibility of subject reports of change in sexual orientation.

“I offered several (unconvincing) reasons why it was reasonable to assume that the subjects’ reports of change were credible and not self-deception or outright lying. But the simple fact is that there was no way to determine if the subject’s accounts of change were valid.”

‘Gay conversion’ therapy, which is popular among Christian conservative groups in the US, has been labelled as “harmful to the psychological well-being of gay people” by the American Psychological Association.