WASHINGTON — The Supreme Court ruled Monday a landmark civil rights law protects gay, lesbian and transgender people from discrimination in employment, a resounding victory for LGBT rights from a conservative court.

The court decided by a 6-3 vote that a key provision of the Civil Rights Act of 1964 known as Title VII that bars job discrimination because of sex, among other reasons, encompasses bias against LGBT workers.

“An employer who fires an individual for being homosexual or transgender fires that person for traits or actions it would not have questioned in members of a different sex,” Justice Neil Gorsuch wrote for the court. “Sex plays a necessary and undisguisable role in the decision, exactly what Title VII forbids.”

Justices Samuel Alito, Brett Kavanaugh and Clarence Thomas dissented.

The outcome is expected to have a big impact for the estimated 8.1 million LGBT workers across the country because most states don’t protect them from workplace discrimination. An estimated 11.3 million LGBT people live in the U.S., according to the Williams Institute at the UCLA law school.

The cases were the court’s first on LGBT rights since Justice Anthony Kennedy’s retirement and replacement by Kavanaugh. Kennedy was a voice for gay rights and the author of the landmark ruling in 2015 that made same-sex marriage legal throughout the United States. Kavanaugh generally is regarded as more conservative.

Efforts by Congress to change the law to explicitly bar job discrimination on the basis of sexual orientation and gender identity have so far failed.

Gov. Tom Wolf, when asked at a news conference Monday about the Supreme Court decision, said, “We still have work to do in nondiscrimination, and I support that.”

The Supreme Court cases involved a transgender woman and two gay men who sued for employment discrimination after they lost their jobs.

Aimee Stephens lost her job as a funeral director in the Detroit area after she revealed to her boss that she had struggled with gender most of her life and told him she would report to work wearing a conservative skirt suit or dress that Rost required for women who worked at his three funeral homes.

Rost fired Stephens. However, the 6th U.S. Circuit Court of Appeals in Cincinnati, Ohio, ruled that the firing constituted sex discrimination under federal law.

That ruling was a victory for the relatives of Donald Zarda, who was fired in 2010 from a skydiving job after trying to put a woman with whom he was tandem jumping at ease by explaining that he was gay. The school fired Zarda after the woman’s boyfriend called to complain.

The federal appeals court in New York ruled in favor of Zarda who claimed he was fired because of his sexual orientation. The full 2nd U.S. Circuit Court of Appeals ruled 10-3 that it was abandoning its earlier holding that Title VII didn’t cover sexual orientation because “legal doctrine evolves.” The court held that “sexual orientation discrimination is motivated, at least in part, by sex and is thus a subset of sex discrimination.”

Lastly, in a case from Georgia, the federal appeals court in Atlanta ruled against Gerald Bostock, a gay employee of Clayton County. Bostock claimed he was fired in 2013 because he is gay. However, county officials argued the firing was the result of an audit of funds he managed.

Associated Press writer Ed White and Butler Eagle staff writer Nathan Bottiger contributed to this report.