Mark Hinkle, the Social Security Administration’s acting press secretary, did not respond to a question about whether the administration was sharing its data with the Immigration and Customs Enforcement agency.

“Social Security is committed to maintaining the accuracy of earnings records used to determine benefit amounts to ensure people get the benefits they have earned,” he said in an emailed statement. “If we cannot match the name and SSN reported on a W-2 to our records, we cannot credit earnings to a worker’s record.”

The administration of President George W. Bush tried, and failed, to introduce a “no-match” program in 2007 that would have held companies liable for employing unauthorized workers by imposing stiff penalties on them. The program was initiated after Congress failed to pass a bill to legalize the nation’s estimated 11 million undocumented immigrants.

But the American Civil Liberties Union, United States Chamber of Commerce, unions and trade groups won a lawsuit later that year that claimed the policy could lead to discrimination against or termination of native-born American workers and legal immigrant workers. The suit also claimed that the regulation would pose a heavy burden on employers.

The latest letters appear to avoid the legal pitfalls identified in the earlier litigation because, unlike those drafted under the Bush program, the current letters do not threaten employers with enforcement action or penalties.

Immigration lawyers have been inundated with inquiries in recent months. Kathleen Campbell Walker, who practices in El Paso, said that one of her clients, a small restaurant chain, could lose a third of its work force. Another, which boasts 50,000 workers in multiple states, had also been alerted of discrepancies by the government.

Jeff Joseph, an immigration lawyer in Denver, said that half of the dairy farms he represents have received no-match letters in the last two months.