By Mary Slosson

SACRAMENTO, Dec 4 (Reuters) - A federal judge on Tuesday cleared the way for a landmark California law that bars a controversial therapy aimed at reversing homosexuality in children and teenagers to take effect in January.

California's Democratic Governor Jerry Brown signed the ban into law in September, making the nation's most populous state the first to ban so-called conversion therapy among youths.

U.S. District Court Judge Kimberly Mueller denied an injunction request against the law filed by the National Association for Research and Therapy of Homosexuality and the American Association of Christian Counselors, as well as unnamed individuals who sued shortly after the law was signed.

"The court finds there is no fundamental or privacy right to choose a specific mental health treatment the state has reasonably deemed harmful to minors," Mueller wrote in a 44-page decision.

The ruling came one day after another federal judge chose to allow an injunction against the law stemming from a different lawsuit, but only applied the ruling to three individuals: Two licensed therapists and one aspiring therapist.

U.S. District Court Judge William Shubb ruled the trio would temporarily not be subject to the legislation pending resolution of a trial on their complaints.

The law bars therapists from performing sexual-orientation change counseling with children and teenagers under 18. It was supported by the California Psychological Association.

Gay rights advocates say the therapy can psychologically harm gay and lesbian youths, leading to depression or even suicide. They say the treatment, also called reparative therapy, has no medical basis because homosexuality is not a disorder.

"On behalf of the untold number of children who can expect to be spared the psychological abuse imposed by reparative therapy, I'm thrilled that today's ruling by Judge Mueller will continue to protect our children from serious harm," the bill's sponsor, state Senator Ted Lieu, said in a statement.

"The net effect of these two rulings is that SB 1172 takes effect Jan. 1 for everyone except for three individuals," he added.

Lawyers for the plaintiffs in both cases, filed against Brown and other state officials, argued that the law violates constitutionally protected rights to free speech and freedom of religion.

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