The Supreme Court on Thursday strengthened protections for religious rights in the workplace, siding in part with a Sabbath-observant mail carrier who quit the U.S. Postal Service after he was forced to deliver packages on Sundays.

Gerald Groff, the former postal worker, had asked the justices to overturn a decades-old Supreme Court decision, which his lawyers said undermines religious protections by allowing employers to deny accommodations that would cause them more than a minor inconvenience.

In a unanimous decision, authored by Justice Samuel A. Alito, the justices declined to get rid of past precedent, and instead clarified that employers must meet a higher standard to reject a workers’ request related to religious observance.

At issue in Groff’s case is title VII of the Civil Rights Act, which prohibits religious discrimination in the workplace and requires employers to reasonably accommodate an employee’s observance unless that accommodation imposes an “undue hardship” on the business. In 1977, the court defined such a hardship as an accommodation that would place more than a minimal burden or “de minimis cost,” on the company’s operations.

Groff’s lawyers asked the court to overrule that decision, Trans World Airlines v. Hardison, which has long drawn criticism from some conservative justices.

Instead, the court clarified Thursday that an undue hardship is “shown when a burden is substantial in the overall context of an employer’s business.”

The Biden administration urged the justices not to toss the earlier decision, which it said has been interpreted by many lower courts and the Equal Employment Opportunity Commission to provide “meaningful protection for religious observance without imposing substantial burdens on employers and co-workers.”

At oral argument in April, a majority of the justices seemed interested in a compromise that would balance religious rights in the workplace with the burden they might impose on employers and co-workers.

Before those arguments, there was reason to think that the court would be receptive to Groff’s position. Three members of the court’s conservative majority — Justices Clarence Thomas, Samuel A. Alito Jr. and Neil M. Gorsuch — had already expressed skepticism about the 1977 decision.

More broadly, the court’s majority has been increasingly protective of religious liberty, handing a number of successes to religious conservatives. Last term, the court sided with a former public high school football coach who was disciplined for postgame prayers at midfield and ruled that the city of Boston was wrong to deny a Christian group’s request to fly its flag at city hall when it had never turned down other organizations.