The Supreme Court ruled unanimously Monday that a group of African Americans may sue the city of Chicago on their claim that the city’s discriminatory use of an application test kept them from being hired as firefighters.

The justices rejected the city’s argument that the class of 6,000 black applicants had waited too long to bring the suit. Antonin Scalia, writing for the court, said each time the city used the test results to hire firefighters over the next six years it represented a new chance for the denied applicants to bring their suit.

“Under the city’s reading, if an employer adopts an unlawful practice and no timely charge is brought, it can continue using the practice indefinitely, with impunity, despite ongoing disparate impact” on minorities, Scalia wrote.

Federal law forbids employers from using an employment practice that “causes a disparate impact on the basis of race, color, religion, sex or national origin,” even if there is no discriminatory intent. The only exception is if the employer can show the practice is job-related and “consistent with business necessity.”

The city gave an entry-level firefighter test that drew 26,000 applicants, 45 percent of them white and 37 percent of them black. The city decided that scoring 65 on the test made an applicant “qualified” for the next step in securing a job.

But faced with a huge number of qualified applicants, the city created two groups: a “well-qualified” set of those who scored 89 or better, and a “qualified” group of those who scored 65 to 88.

For years, the city limited its hiring to the “well-qualified” group, which was skewed racially — 76 percent were white and 11.5 percent were black. In all, 10 classes of applicants were drawn from the group, and the city hired some from the second group only after the first was exhausted.

Thousands of black applicants who were deemed “qualified” sued and won. A federal judge said the city had known that the 89 cutoff score was “statistically meaningless” and that there was no proof that those who scored higher on the test made better applicants.

The judge ruled that each time the city hired from the list, it constituted a “fresh act of discrimination.”