Employers are not permitted to use a worker’s prior pay to justify compensating female and male employees differently for the same work, a federal appeals court ruled Thursday in a closely watched case confronting the contours of the federal Equal Pay Act.

The Equal Pay Act, or EPA, “requires employers to demonstrate that only job-related factors, not sex, caused any wage disparities that exist between employees of the opposite sex who perform equal work,” Judge Morgan Christen of the U.S. Court of Appeals for the Ninth Circuit wrote in the case Rizo v. Yovino, where the Fresno County school district was sued for alleged pay discrimination.