Dr Robert Spitzer, hailed as the "most influential psychiatrist" has died of heart problems in Seattle on Friday 25 December, said his wife. Spitzer played a major role in removing homosexuality's classification as a mental disorder in 1973.

Prior to this date the Diagnostic and Statistical Manual of Mental Disorders (DSM) had classified homosexuality as an illness, designating it a "sociopathic personality disturbance."

Spitzer became convinced that declassification was necessary after meeting with gay activists and determined that homosexuality could not be a disorder if gay people were comfortable with their sexuality.

"A medical disorder either had to be associated with subjective distress, pain or general impairment in social function," he told the Washington Post.

Dr Jack Drescher, a gay psychoanalyst, described it as paving the way forward for gay rights."The fact that gay marriage is allowed today is in part owed to Bob Spitzer," he told the New York Times.

Ronald Bayer of the Center for the History and Ethics of Public Health at Columbia said: "I wouldn't say that Robert Spitzer became a household name among the broader gay movement, but the declassification of homosexuality was widely celebrated as a victory."

"Bob Spitzer was by far the most influential psychiatrist of his time," Dr. Allen Frances, a professor emeritus of psychiatry at Duke University, told the New York Times. "He saved the field and its millions of patients from a crisis of credibility, raising its scientific standards and rescuing it from the arbitrariness of warring and unsupported opinions."

However, Spitzer caused controversy after publishing a study in 2001 which appeared to support reparative therapy, also known as conversion therapy, which claims to be effective in turning gay people into heterosexuals. But he realised that the study was flawed because of the wording of the study which asked if those who had reparative therapy had changed their sexual orientation.

"As I read these commentaries [about the study], I knew this was a problem, a big problem, and one I couldn't answer," Spitzer told the Times. "How do you know someone has really changed?"

In a letter to Kenneth J Zucker, editor of Journal of Sexual Behaviour, Spitzer wrote:

"Several months ago I told you that because of my revised view of my 2001 study of reparative therapy changing sexual orientation, I was considering writing something that would acknowledge that I now judged the major critiques of the study as largely correct. After discussing my revised view of the study with Gabriel Arana, a reporter for American Prospect, and with Malcolm Ritter, an Associated Press science writer, I decided that I had to make public my current thinking about the study. Here it is.

"Basic Research Question. From the beginning it was: "can some version of reparative therapy enable individuals to change their sexual orientation from homosexual to heterosexual?" Realizing that the study design made it impossible to answer this question, I suggested that the study could be viewed as answering the question, "how do individuals undergoing reparative therapy describe changes in sexual orientation?" – a not very interesting question.

"The Fatal Flaw in the Study – 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 subject's 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.

"I believe I owe the gay community an apology for my study making unproven claims of the efficacy of reparative therapy. I also apologize 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."