Denver will now require that contractors on city construction projects check the immigration status of their employees.

The city council gave final approval to the measure Monday night.

It requires companies use the federal E-Verify program to check whether workers are in the country legally. The program works by checking information collected by an employer against data from the U.S. Department of Homeland Security and the Social Security Administration.

Denver already requires E-Verify for city service contracts.

Opponents of the program say it isn’t always accurate and could cause people who are in the country legally to be mistaken for illegal workers. Councilors added an appeals process to the measure before passing it.