WASHINGTON — The Supreme Court declared Monday that the historic Civil Rights Act of 1964 protects LGBTQ employees from workplace discrimination.

In a major victory for lesbian, gay, bisexual, transgender and queer workers, the justices said the law’s ban on job discrimination on the basis of “sex” can be read to forbid bias against employees because of their sexual orientation or gender identity.

Justice Neil M. Gorsuch, appointed by President Donald Trump, spoke for a 6-3 majority that included Chief Justice John G. Roberts Jr. and the court’s liberal justices.

The ruling delivers one of the most far-reaching civil rights advances in recent decades. While California and at least 20 other states have protected LGBTQ employees from workplace discrimination, states in the South and most of the Midwest lack such laws.

Democrats in the House have passed bills to protect lesbian, gay, bisexual, transgender and queer people from discrimination, but the Republican-controlled Senate has refused to take up the measures.

Now the high court has done so on its own, based on a strict reading of the 1964 law. The outcome defied the common prediction that the court would turn far more conservative on gay rights following the retirement of Justice Anthony M. Kennedy, the Republican appointee who wrote the court’s major gay rights decisions, including the 2015 ruling upholding same-sex marriages.

The decision is a remarkable example of a conservative justice reinterpreting the words of a landmark liberal law. Gorsuch, a former clerk for Kennedy, acknowledged the law was not originally intended to protect gay and lesbian employees. But he concluded that its words prohibited employers from making hiring or firing decisions based not only on gender, but also on sexual orientation and gender identity.

“Today, we must decide whether an employer can fire someone simply for being homosexual or transgender,” Gorsuch wrote. “The answer is clear. 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. Sex plays a necessary and undisguishable role in the decision, exactly what Title VII forbids.

“Those who adopted the Civil Rights Act might not have anticipated their work would lead to this particular result. Likely, they weren’t thinking about many of the act’s consequences that have become apparent over the years, including its prohibition against discrimination on the basis of motherhood or its ban on the sexual harassment of male employees. But the limits of the drafters’ imagination supply no reason to ignore the law’s demands. When the express terms of a statute give us one answer and extratextual considerations suggest another, it’s no contest. Only the written word is the law, and all persons are entitled to its benefit.”

In addition to the chief justice and Gorsuch, the decision was joined by Justices Ruth Bader Ginsburg, Stephen Breyer, Sonia Sotomayor and Elena Kagan. In dissent were Justices Clarence Thomas, Samuel A. Alito Jr. and Brett M. Kavanaugh.

The ruling came as a shock to many, even though Gorsuch had signaled during the oral argument in October that the case was “really close.” Gorsuch had promised in his confirmation hearings that he would follow the words of the law and be guided by what it said, not by his view of a good policy or the intentions of the legislature. And these cases fit that model.

Roberts’ decision to join in the ruling was also a surprise. He had strongly dissented when the court upheld same-sex marriages.

While Monday’s decision is a landmark victory for LGBTQ employees, it was based on the court’s legal interpretation of an existing law passed by Congress, not constitutional guarantees, as was the case with several previous rulings on LGBTQ rights.

It also does not guarantee that the high court will be friendly forum for gay rights in the future. Gorsuch and the court’s conservatives are also champions of religious rights, and they have signaled a willingness to look favorably on claims from religious-minded people who refuse to participate in same-sex marriages or work with gay couples in arranging adoptions or foster care. Future cases will be instrumental in forging the balance between gay rights and religious liberties.

The dispute arose after Gerald Bostock was fired from his county job as a child welfare advocate shortly after he joined a gay softball league. He lost his case in federal courts in Georgia. His case was combined with two others in which courts in New York and Michigan ruled for gay or transgender employees.

The two other plaintiffs have passed away. Donald Zarda was a sky diving instructor in New York who was fired after telling a customer he was gay. He later died in a sky diving accident. Aimee Stephens had worked at a funeral home in Detroit, but was fired after she returned to work as a woman. She died last month.

Alito wrote a long dissent, joined by Thomas, that condemned the decision as judicial lawmaking. “There is only one word for what the court has done today: legislation,” Alito wrote.

Alito predicted the court’s decision would impact numerous pending legal battles over rights for transgender people, including disputes over coverage exclusions in health plans and use of school bathrooms.

“Although the court does not want to think about the consequences of its decision, we will not be able to avoid those issues for long,” Alito wrote. “The entire federal judiciary will be mired for years in disputes about the reach of the court’s reasoning.”

Kavanaugh, in a separate dissent, took a milder tone. He said it was up to Congress to pass such protections, not the court. But he signaled some support for the outcome.

“Notwithstanding my concern about the court’s transgression of the Constitution’s separation of powers, it is appropriate to acknowledge the important victory achieved today by gay and lesbian Americans,” he wrote. “Millions of gay and lesbian Americans have worked hard for many decades to achieve equal treatment in fact and in law. They have exhibited extraordinary vision, tenacity, and grit —battling often steep odds in the legislative and judicial arenas, not to mention in their daily lives. They have advanced powerful policy arguments and can take pride in today’s result.”

Trump, whose administration has moved aggressively to curtail transgender rights, did not immediately comment about the ruling. The administration joined the case to argue for the employers and against protecting the LGBTQ employees from discrimination.

Democratic presidential hopeful Joe Biden called the ruling “a momentous step forward for our country.”