The US Supreme Court rejected on Tuesday a Colorado law that banned psychotherapists from using "conversion" talk therapy intended to change an LGBT minor's sexual orientation or gender identity, siding with a Christian licensed counselor in casting the prohibition as an intrusion on free speech rights.

The 8-1 ruling, authored by conservative Justice Neil Gorsuch, rejected Colorado's argument that its law regulated professional conduct, not protected speech.

The justices reversed a lower court's decision that had upheld the law in a case brought by counselor Kaley Chiles, who argued that it violated the US Constitution's First Amendment protections against government abridgment of free speech.

The Supreme Court held open the possibility that the law could apply to certain forms of conversion therapy, including so-called "aversive" physical interventions, but not to the counselor's speech at issue in the case.

"Colorado's law addressing conversion therapy does not just ban physical interventions. In cases like this, it censors speech based on viewpoint," Gorsuch wrote. "Colorado may regard its policy as essential to public health and safety. Certainly, censorious governments throughout history have believed the same. But the First Amendment stands as a shield against any effort to enforce orthodoxy in thought or speech in this country."

The justices directed the lower court to conduct further proceedings applying a more rigorous First Amendment standard to the law.

US Republican President Donald Trump's administration backed Chiles in the challenge to the law.

The Supreme Court has a 6-3 conservative majority, and liberal Justice Ketanji Brown Jackson was the lone dissenter.

The dispute pitted Colorado's authority to forbid a healthcare practice that it called unsafe and ineffective against First Amendment speech protections.

Jackson said that states have the power to regulate medical treatments provided by state-licensed professionals to patients, and Colorado's decision to "restrict a dangerous therapy" that incidentally involves providers' speech is not unconstitutional.

"In concluding otherwise, the court's opinion misreads our precedents, is unprincipled and unworkable, and will eventually prove untenable for those who rely upon the long-recognized responsibility of states to regulate the medical profession for the protection of public health," Jackson wrote.

Colorado is among more than two dozen states and the District of Columbia that restrict or prohibit conversion therapy for patients younger than 18.

Democratic Colorado Governor Jared Polis, the first openly gay man to be elected as a US state governor and a critic of conversion therapy, signed the measure into law in 2019. Polis said on Tuesday he was evaluating the ruling and working to figure out how to better protect LGBT youths and free speech in Colorado.

"Conversion therapy doesn't work, can seriously harm youth, and Coloradans should beware before turning over their hard-earned money to a scam," Polis said. "We are fighting for everyone's right to be who you are in our Colorado for all."

The law prohibited licensed mental healthcare providers from seeking to change a minor's sexual orientation or gender identity according to a predetermined outcome, with each violation punishable by a fine of up to $5,000. This includes attempts to reduce or eliminate same-sex attraction or change "behaviors or gender expressions."

Medical groups such as the American Psychological Association have cited studies showing that this type of talk therapy has been associated with harms including an increased likelihood of transgender minors attempting suicide or running away from home.

Colorado's law does permit treatments that provide "assistance to a person undergoing gender transition," as well as therapies centered on "acceptance, support and understanding" for "identity exploration and development."

Chiles, a practicing Christian, has said she "believes that people flourish when they live consistently with God's design, including their biological sex." Chiles was represented by the Alliance Defending Freedom, a conservative religious rights group that previously secured high-profile Supreme Court victories on behalf of a baker and wedding website designer who refused, based on their Christian beliefs, to serve gay couples.

Alliance Defending Freedom lawyer Jim Campbell welcomed Tuesday's ruling.

"Kids deserve real help affirming that their bodies are not a mistake and that they are wonderfully made. The US Supreme Court's decision today is a significant win for free speech, common sense, and families desperate to help their children," Campbell said. "States cannot silence voluntary conversations that help young people seeking to grow comfortable with their bodies."

Kelley Robinson, president of the Human Rights Campaign LGBT rights group that filed a brief in support of the law, said the Supreme Court's "reckless decision means more American kids will suffer."

"The court has weaponized free speech in order to prioritize anti-LGBTQ+ bias over the safety, health and wellbeing of children. So-called 'conversion therapy' is pseudoscience, not real therapy," Robinson said.

"We remain committed to protecting children against these abusive practices that tear apart families and will continue to work alongside our coalition partners to ensure that no kid has to be subjected to guilt, coercion and rejection as they seek help to better understand themselves and grow," Robinson added.