The approach is expected to play well with conservatives who have long demanded that the administration do more to enforce existing immigration laws, but it could also lead to renewed pressure from businesses on Congress to provide legal status for an estimated six million unauthorized immigrant workers.

“We are tough and we are going to be even tougher,” Russ Knocke, the spokesman for the Department of Homeland Security, said yesterday. “There are not going to be any more excuses for employers, and there will be serious consequences for those that choose to blatantly disregard the law.”

Experts said the new rules represented a major tightening of the immigration enforcement system, in which employers for decades have paid little attention to notices, known as no-match letters, from the Social Security Administration advising that workers’ names and numbers did not match the agency’s records.

Illegal workers often provide employers with false Social Security numbers to qualify for a job.

Employers, especially in agriculture and low-wage industries, said they were deeply worried about the new rules, which could force them to lay off thousands of immigrant workers. More than 70 percent of farmworkers in the fields of the United States are illegal immigrants, according to estimates by growers’ associations.

“Across the employer community people are scared, confused, holding their breath,” said Craig Regelbrugge, co-chairman of the Agriculture Coalition for Immigration Reform, a trade organization. “Given what we know about the demographics of our labor force, since we are approaching peak season, people are particularly on edge.”