A federal judge in San Francisco ordered an indefinite delay yesterday of a central measure of the Bush administration’s new strategy to curb illegal immigration.

A federal judge in San Francisco ordered an indefinite delay yesterday of a central measure of the Bush administration’s new strategy to curb illegal immigration.

The judge, Charles R. Breyer of the Northern District of California, said the government had failed to follow proper procedures for issuing a new rule that would have forced employers to fire workers if their Social Security numbers could not be verified within three months.

Judge Breyer chastised the Department of Homeland Security for making a policy change with “massive ramifications” for employers, without giving any legal explanation or conducting a required survey of the costs and impact for small businesses.

Under the rule issued by the department, which had been scheduled to take effect last month, employers would have to fire workers within 90 days after receiving a notice from the Social Security Administration that an employee’s identity information did not match the agency’s records. Illegal immigrants often present false Social Security information when applying for jobs.

The rule, announced with fanfare in August by Homeland Security Secretary Michael Chertoff, was the linchpin of the administration’s effort to crack down on illegal immigration by denying jobs to the immigrants. It is part of a campaign of stepped-up enforcement since broader immigration legislation favored by President Bush was rejected by Congress in June.

If allowed to take effect, the judge found, the rule could lead to the firing of many thousands of legally authorized workers, resulting in “irreparable harm to innocent workers and employers.”

The decision brought a sense of relief to the unusual coalition behind the lawsuit, including the A.F.L.-C.I.O. and the United States Chamber of Commerce, often adversaries. They had feared that the measure would bring mass layoffs in low-wage industries, sweeping up both illegal and legal workers and disrupting the labor force.