A U.S. government plan to crack down on employers who knowingly hire illegal immigrants was delayed again on Monday as a federal judge said he needed more time to issue a ruling.

The Social Security Administration and the Department of Homeland Security were to send joint letters in September warning businesses they will face penalties if they keep workers whose Social Security numbers do not match their names.

The restraining order was necessary in the meantime because "it's clear ... there could be irreparable harm to plaintiffs" if the government went ahead with its plan as it stands, U.S. District Judge Charles Breyer said.

The new rule would require businesses to sort out any Social Security mismatches within 90 days of being notified by the government or to fire the employee involved.

The government has 140,000 such letters ready to go as soon as the judge grants permission, Deputy Assistant Attorney General Tom Dupree said.

Lawyers for the plaintiffs, which include the AFL-CIO trade union federation, the American Civil Liberties Union, the U.S. Chamber of Commerce and numerous other business and labor groups, claim the rule would put a heavy burden on employers.

It also could cause many authorized immigrants and U.S. citizens to lose their jobs over innocent paperwork snafus, they said. Employers who were trying to comply with the government's mandate could in turn be exposed to discrimination lawsuits brought by employees who got wrongfully terminated.

The plan, unveiled in August, is an example of a government agency overstepping its jurisdiction, said ACLU attorney Lucas Guttentag.

"The Department of Homeland Security does not have the authority to hijack the Social Security Administration's data for immigration enforcement," Guttentag said. "We believe this rule is a massive violation of the law."

Immigration control advocates, meanwhile, remained hopeful the judge would see this rule as they do -- as a late, but welcome effort to enforce immigration laws that have been on the books, but overlooked for years.

"It's those people who want to continue to use cheap, illegal labor that do not want this rule enforced," said Rick Oltman, with Californians for Californians for Population Stabilization.