And if that doesn't clear up the situation, employers must either terminate the employee or risk being the target of a government probe that could find them responsible for employing an undocumented worker -- a violation of federal law.

The regulation, which makes the employer more responsible for verifying an employee's legal status than in the past, is the latest in a string of government laws and rules aimed at business that grew out of the heightened security awareness after the Sept. 11 attacks as well as calls for immigration reform.

"The effects of 9/11 are pervasive," said Gregory Mark, a professor at Rutgers Law School in Newark who specializes in business law.

Banks and financial institutions, for instance, now have to more closely track money transfers and deposits -- measures designed to help authorities crack down on the funding of terrorist groups, he said.

Immigration lawyers say the visa-application procedure that enables companies to hire foreign employees is now more lengthy and laden with red tape. David Nachman, a Ridgewood immigration attorney, said the U.S. consulates abroad now conduct particularly rigorous background checks on workers looking to come to the U.S.

"What we are seeing now is a lot more enforcement," he said, noting that the change began when the Homeland Security Department took responsibility for enforcing U.S. immigration laws in 2003. "It's more delay, more bureaucracy. For business, it means you have to dot your i's, cross your t's."

Another, newer regulation requires importers to tell U.S. customs officials what's in a shipment destined for the U.S. 24 hours before it sets out, a measure designed to stop such shipments being used to smuggle terrorist weapons.

And the USA Patriot Act., which was created soon after 9/11 to help the government more easily access information for tracking terrorists, requires Internet providers, phone companies and other businesses to provide customer information to federal authorities.

A federal judge last week struck down parts of the act, saying investigators must have a court's approval before they can order Internet providers to turn over records without telling customers.

Blake Ellman, CEO of Net Access Corp., a Cedar Knolls-based Internet provider and Web site hosting company, said he receives four to six search warrants a year from federal authorities seeking customer information for terrorist-related investigations. That requires his employees to work with federal agents to dig out and copy required documents and data, and sometimes help them set up monitoring equipment for particular accounts, he said.

"The burden for us from an economic standpoint is we have to comply with these things and we don't get paid," he said. "From a labor standpoint, it eats away productive time."

He said that in the post-9/11 world he's also had to upgrade his record-keeping procedures, and conduct more vigorous background checks on new employees, to meet the demands of customers, such as financial institutions, with special security concerns.

Immigration attorneys say the no-match regulation, though created by the Homeland Security Department, was largely fueled by the country's preoccupation with how to handle undocumented immigrants. Yet some politicians say the ability of federal authorities to closely track immigrants within the U.S. border is a key element of national security.

The regulation was due to take effect this Friday. But a federal judge in California delayed the implementation until a court hearing on the issue Oct. 1. The AFL-CIO had filed suit to stop the regulation, saying it could be used to discriminate against some workers. On Monday the U.S. Chamber of Commerce said it had joined the suit.

Attorneys who represent businesses in immigration matters say their clients see the regulation as an added burden fraught with pitfalls.

The attorneys say the Social Security Administration has issued no-match letters for years, and while some employers responded others ignored the letters, knowing that the agency took few enforcement actions.

Nachman, the Ridgewood immigration attorney, said the reason an employee's Social Security number doesn't match the federal records may have nothing to do with the worker's immigration status. For instance, the person may have changed her name due to marriage and not informed authorities, or the name may not have been input correctly in the federal database, he said.

That's particularly problematic with Hispanic employees, who often use several surnames, Nachman said. He said one of his clients, which employed a lot of Hispanics, was informed that a third of the company's workforce had names that did not match the federal records.

That's troublesome because the regulation would make employers more responsible for knowing their employees' legal status, said Edwin R. Rubin, a Newark immigration attorney. It creates a lot of work for human resources employees, especially at companies that have high worker turnover, he said. And some employers may just say they don't need the headache, he added.

"We are concerned that this may lead to selective discrimination, that employers will stop hiring people that may look and sound foreign," he said.