Last night, the Denver City Council approved a measure requiring construction contractors to use the federal E-Verify system to check the immigration status of employees. The 9-4 vote also requires firms doing business with the city to affirm, as of October 1, that their existing employees are able to work legally in the United States. “No citizen or legal resident should have to compete with workers who are here illegally,” says Councilman Charlie Brown, pictured (via The Denver Post). The new rule adds to a city requirement that companies use the E-Verify online database to check immigration documentation with the U.S. Department of Homeland Security and the Social Security Administration.

Discussion before the measure passed on Monday focused on using a system that is not proven to be 100 percent accurate in determining the eligibility status of workers across the country, reports the Denver Business Journal. In Arizona, where E-Verify is required, attorney David Selden, who represents the Arizona Chamber of Commerce and other business organizations, is expected to bring the issue to the U.S. Supreme Court, according to Fox 10. Under the law there, a business owner can be prosecuted for knowingly hiring an undocumented person. But the “E-Verify system is still a pilot program. It’s not accurate; it’s not ready for prime time,” he says.