The federal government is exploring the possibility of using a credit rating giant like Equifax to verify the identity of American workers, a move that could make it far more difficult for undocumented immigrants to get work using stolen Social Security numbers.

The plan by the Department of Homeland Security, which is still preliminary and would probably require congressional approval, could have far-reaching consequences. The government already allows employers to check the legal status of employees using a system known as E-Verify, but hundreds of thousands of undocumented immigrants beat the system by using stolen Social Security numbers.

If workers had to use the verification systems in place to apply for a mortgage or a bank account, they would not only have to present a Social Security number to an employer, but also answer questions about their personal history and financial background to establish their identity.

On Monday, the government announced that it would begin allowing individuals in the District, Virginia and four other states to voluntarily use a system provided by Equifax to verify their identity. Once they did that, they could access a federal database to verify their authorization to work. The move will help the small number of legally authorized immigrants and U.S. citizens who encounter problems each year when an employer runs their Social Security numbers through the E-Verify system.

By giving workers the ability to check their records before they apply for a job, authorities said that citizens and immigrants who are authorized to work will be able to take care of spelling mistakes and other common errors. The voluntary program will be piloted in the District, Virginia, Arizona, Colorado, Idaho and Mississippi. It will be expanded nationwide in the coming months.

Alejandro Mayorkas, director of U.S. Citizenship and Immigration Services, said the government planned to use the initiative to evaluate how the third-party verification system worked, with a view to making the tool available to employers.

Mayorkas added that only Congress could compel employers to use third-party verification systems. The main E-Verify system is also voluntary for employers, but House Republicans have indicated that they would like it to be mandatory.

Private identification systems might reduce Social Security number fraud, but Mayorkas said he has concerns about how the federal government would deal with errors in third-party databases.

Neither employers nor the federal government will gain information about worker queries under the new self-check system. Mayorkas also said that employers will not be permitted to force employees to do self-checks.