SAN FRANCISCO - A federal judge on Wednesday granted a request by labor and civil liberties organizations to temporarily block the U.S. government from proceeding with a plan to crack down on businesses that may be employing illegal immigrants.

U.S. District Judge Charles Breyer said the Social Security Administration and the Department of Homeland Security could not go ahead with a plan to send joint letters warning businesses they'll face penalties if they keep workers whose Social Security numbers don't match their names.

Breyer said the new work site rule would likely impose hardships on businesses and their workers.

"The plaintiffs have demonstrated they will be irreparably harmed if DHS is permitted to enforce the new rule," Breyer wrote.

The so-called "No Match" letters were supposed to start going out in September, but labor groups and immigrant activists filed a lawsuit claiming the plan would put a heavy burden on employers and could cause many authorized immigrants and U.S. citizens to lose their jobs over innocent paperwork glitches.

The government argued that the rule doesn't impose an expense, and some businesses want to avoid liability for hiring undocumented workers.

Rep. Brian Bilbray, R-Solana Beach, criticized the judge's decision, saying the ruling would make it harder for federal authorities to enforce immigration laws.

"Using a Social Security number that does not belong to you is a felony," Bilbray said in a written statement. "Judge Breyer is compromising the rule of law principles that he took an oath to uphold. At a time when the federal government is finally trying to enforce current immigration law, we cannot have activist judges stand in the way of doing what is right."

Lilia Velasquez, a San Diego immigration attorney, said the ruling will not prevent authorities from enforcing immigration laws.

"If (the Department of Homeland Security) has information regarding a no-match number for someone who claims to be in legal status, they will simply request an audit of the employee records from the employers, and then make a determination as to whether they will arrest the undocumented worker," Velasquez said.

On Oct. 1, Breyer requested time to consider the legal arguments presented by government attorneys and plaintiffs, which include the AFL-CIO, the American Civil Liberties Union, the U.S. Chamber of Commerce and a number of other business and labor groups.

A large portion of the mismatches in the Social Security Administration's records are believed to stem from illegal immigrants who make up Social Security numbers to get a job.

In August, Homeland Security Secretary Michael Chertoff announced the agency would start notifying businesses that if workers were unable to clear up problems with their Social Security numbers within 90 days, they would have to be terminated. If not, their employer could face criminal fines and other sanctions.

According to government attorneys, the rule simply clarifies employers' obligations under immigration law and gives them clear guidelines on how to handle mismatched records. Until now, the Social Security Administration regularly notified employers of discrepancies, but employers were not required to act.

The measure was scheduled to take effect in September, with the mailing of about 140,000 letters, each containing the names of 10 or more employees with mismatches in their records.

The San Francisco federal court blocked its implementation after the suit was filed, issuing temporary restraints until the judge could determine whether the plaintiffs would suffer damage if the government were allowed to go forward with its plan.

Attorneys representing businesses associations argued the Department of Homeland Security's plan would place a costly burden on them that could lead to the needless firing of employees. That, in turn, would open them up to lawsuits and charges of discrimination.

Civil liberties organizations joining the suit pointed out the rule likely would lead to the violation of the rights of many legal workers who might have made a mistake they couldn't correct before deadline.

An analysis of the "No Match" letter policy estimated that at least 1.5 million workers would be affected by the letters, a quarter of whom live in California, according to the Migration Policy Institute, a Washington-based think tank that specializes in immigration matters.

"The Social Security Administration is prohibited from releasing no-match information to the Department of Homeland Security for enforcement purposes. So the regulation would not make it any more likely that an employer who receives a no-match letter would be the target of an investigation. Instead, the initiative attempts to lead employers to take action themselves," said Doris Meissner, a researcher at the institute.

- Staff writer Edward Sifuentes contributed to this story.