It is believed that the clinic is not alone in its offer of such treatment in Spain.
Speaking to the daily newspaper Periodico de Catalunya, psychiatrist Joaquin Munoz, who works at the clinic in question, angered gay rights groups with his defence.
He said, "Nobody wants to be a homosexual, it just happens to people. If [gay people] could change their sexual orientation with a pill, 99 per cent would take it."
Catalan regional Health Minister Marina Geli said there was no evidence of homosexuality being a disease.
She added that the clinic under investigation would face a fine if it was found to be conducting the "treatments".
This is a curious occurence in a country that has stormed ahead of its European neighbours in terms of gay rights, despite being a predominatly Catholic country.
In 2005, Prime Minister Jose Luis Rodriguez Zapatero's socialist government was among the world's first to grant same-sex couples full marriage rights and the right to adopt children.
More than 15,000 gay couples have wed since the reform, which faced vehement opposition from the country's Catholic Church. Spain also allows trans men and women to change their names prior to undergoing sex change operations, which some regions even finance from public money.
This news however, seems closer in spirit to that of the Franco era, during which homosexuality was illegal and many gay men and women were institutionalised.
Many of the gay people believed to be seeking such treatments are thought to have been indoctrinated by the Catholic Church.
A 35-year-old man told El Pais newspaper that he had once undergone a "cure" treatment that included weekly therapy sessions and spending time with people claiming to be "ex-gays" three years ago.
"It amounted to brainwashing," he said. "I thought I was sick and felt guilty."
Psychologist Silvia Morell said, "Homosexuality cannot be cured," adding that any attempts to do so caused "depression, self-destructive behaviour, anxiety and can lead to suicide".
However, a top Catalan politician defended the position of the Barcelona clinic. Josep Antoni Duran Lleida said, "Why should homosexuals not be allowed to try to change in a society which sees sexuality as a changeable . . . option".
Mr Duran Lleida denied that he viewed homosexuality as a disease, but his comments nevertheless angered left-wing groups who decried them as "backward".
Jordi Arcarons, spokesman for the gay association Convergais said, "Being homosexual is not something one chooses or suffers from . . . "one simply is gay, and that is it."
It was only in 1990 that the World Health Organisation removed homosexuality from its list of disorders.
SIC: PNUK