Employers must fire workers who use false Social Security numbers, under new federal rules, which were expected to be announced this week.

The regulations would give employers a fixed period to resolve any discrepancies between identity information provided by their workers and the records of the Social Security Administration. If workers’ documents cannot be verified, employers would be required to fire them or risk up to $10,000 in fines for knowingly hiring illegal immigrants. Nursing homes, which employ many low-wage workers, could be a target of this tightened immigration policy.

Labor groups, including the A.F.L.-C.I.O., said they were preparing legal challenges to try to stop the new rules from taking effect.