A federal judge has blocked a proposed rule requiring employers to fire workers whose names don't match their Social Security numbers, dealing a major blow to the Bush administration's crackdown on illegal immigration.

Under the rule, businesses with employees whose names and Social Security numbers didn't match would have three months to correct the mistakes or fire the employees. If not, they could face government prosecution.

Businesses had argued that enforcing the rule would be expensive and expose them to legal action either from the government, if they didn't comply, or from any employees fired unfairly because of a mistake not corrected in time.

U.S. District Judge Charles Breyer's temporary injunction, issued Wednesday, stopped the Department of Homeland Security proposal from going into effect, at least temporarily.

Breyer said the proposal would likely impose hardships on businesses and their workers. Employers would incur new costs to comply with the regulation that the government hasn't evaluated, and innocent workers unable to correct mistakes in their records in time would lose their jobs, the judge wrote.

The decision was disappointing, said Homeland Security Secretary Michael Chertoff, but it wasn't more than a "bump in the road" in the agency's drive to vigorously enforce laws aimed at keeping illegal immigrants out of the work force.

The government will evaluate the "modest legal obstacles" presented by the judge, addressing them in litigation or outside court, as it examines its options and determines whether to appeal the decision, Chertoff said.

"I don't think there's anything in the judge's ruling that is insurmountable," Chertoff told The Associated Press in a phone interview. "The key is to move forward. We're committed to using every tool available to enforce our immigration laws."

But plaintiffs, including the AFL-CIO, the American Civil Liberties Union and the U.S. Chamber of Commerce, saw the decision as a significant victory against a program they believe would foster discrimination on the work site, lead to job losses by lawful employees, and expose businesses to additional expenses and the fear of prosecution.

"Judge Breyer's decision today reassures authorized workers and U.S. citizens that their rights will be protected," said Marielena Hincapie, with the National Immigration Law Center, an attorney on the case.

The plan to issue Department of Homeland Security warnings was announced in August. The government had about 140,000 letters ready for employers by September, when they were scheduled to be released.

Each contained the names of 10 or more employees with mismatches in their records. About 8 million employees would be affected, according to court documents.

"This would cost a lot of money for employers to comply with," said Angelo Amador, director of immigration policy for the U.S. Chamber of Commerce, a party to the suit. "This was drafted and published in haste, and now the administration is paying for it."

The letters were held after labor groups and immigrant activists filed a federal lawsuit, and asked the judge to block the regulation until its legal merits could be evaluated in court.

The injunction blocks the implementation of the government's plan until the lawsuit is resolved or an appeals court overturns the judge's decision.

For years, the Social Security Administration has let businesses and employees know when there were inconsistencies with their records, mainly so the workers wouldn't lose access to the funds they were contributing to Social Security.

Often, the mismatches stem from undocumented immigrants who make up Social Security numbers to get a job. But because the discrepancies can result from a number of innocent mistakes — misspellings of a name, typos — the agency's letters always clarified that the mailings did not "make any statement about an employee's immigration status."