SAN FRANCISCO - The Social Security Administration cannot start sending out letters to employers next week containing notification of more serious penalties for knowingly hiring illegal immigrants, a federal judge ruled Friday.

Ruling on a lawsuit by the nation's largest federation of labor unions against the U.S. government, U.S. District Judge Maxine Chesney granted a temporary restraining order prohibiting the so-called "no-match" letters from going out as planned starting Tuesday.

The AFL-CIO lawsuit, filed this week, claims that new Department of Homeland Security rules outlined in accompanying letters threaten to violate workers' rights and unfairly burden employers. Chesney said the court needs "breathing room" before making any decision on the legality of new penalties aimed at cracking down on the hiring of illegal immigrants.

She set the next hearing on the matter for Oct. 1.

The Social Security Administration has sent out "no-match" letters for more than two decades warning employers of discrepancies in the information the government has on their workers. Employers often brushed aside the letters, and the small fines that sometimes were incurred, as a cost of doing business.

But this year, those letters will be accompanied by notices from the Department of Homeland Security outlining strict new requirements for employers to resolve those discrepancies within 90 days or face fines or criminal prosecution, if they're deemed to have knowingly hired illegal immigrants.

The judge's ruling Friday temporarily prohibits the government from enforcing the new rules, which were scheduled to take effect Sept. 14.