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SAN FRANCISCO — Relying on a woman's previous salary to determine her pay for a new job perpetuates disparities in the wages of men and women and is illegal when it results in higher pay for men, a federal appeals court ruled Monday.

The unanimous ruling by an 11-judge panel of the 9th U.S. Circuit Court of Appeals came in a lawsuit filed by a California school employee who learned over lunch with colleagues in 2012 that she made thousands of dollars less than her male counterparts.

The 9th Circuit held that pay differences based on prior salaries are inherently discriminatory under the federal Equal Pay Act because the previous salaries were the result of gender bias.

"Women are told they are not worth as much as men," Judge Stephen Reinhardt wrote before he died last month. "Allowing prior salary to justify a wage differential perpetuates this message, entrenching in salary systems an obvious means of discrimination."

The ruling came the day before Equal Pay Day, April 10, which is the symbolic day of the year an average woman must work until she earns the same amount the average man earned the previous year. The 2018 wage gap between women and men is 80 cents — although the gap is far wider for women of color, according to EqualPayToday.org.

Debra Katz, an employment attorney in Washington, D.C., who handles equal pay lawsuits, said the ruling undercut one of the key arguments that employers have made for allowing pay disparities to continue.

"Employers always point to salary history to justify paying an employee less, which just institutionalizes the discrimination," she said.