WASHINGTON — The Bush administration, in an aggressive new effort to keep illegal immigrants out of the U.S. work force, on Monday ordered all companies doing business with the federal government to begin ensuring their employees can work here legally.

The order will require thousands of companies to use a government verification system to check the validity of workers' Social Security numbers. Use of the screening system currently is voluntary for private enterprises but mandatory for government agencies.

The policy initially would apply to new workers but eventually could affect millions of federal contract workers whose jobs range from serving cafeteria food to launching NASA spacecraft. The step is one of several the administration planned after Congress failed last year to pass an overhaul of the nation's immigration laws.

"The federal government should lead by example and not by exhortation," said Homeland Security Chief Michael Chertoff, who has been encouraging businesses to use the voluntary "E-Verify" screening system.

E-Verify has been embraced by groups advocating immigration restrictions as a way to weed illegal workers out of the labor force. But it has been criticized by immigration advocates and business groups because of errors within the Social Security database that can lead to false rejections of legal residents and citizens.

And with the rapid expansion in the use of federal contract workers under the George W. Bush administration, some critics questioned whether the order would be workable.

"I just don't know how the administration is going to enforce this," said Paul C. Light, a New York University professor and federal contracting expert who said such outsourcing has grown by 70 percent since President Bush took office. "It's a very large number and very difficult to track. Who is responsible for making sure the sub, sub, sub-contractor is using E-Verify?"

Chertoff said E-Verify has been a success and predicted that the executive order would affect "hundreds of thousands, if not millions of workers."

Chertoff made the announcement during an appearance with U.S. Commerce Secretary Carlos Gutierrez in which they touted the administration's progress in enforcing immigration laws and beefing up border security. They also pleaded for congressional passage of a comprehensive immigration overhaul that includes guest-worker programs and enforcement, as well as some accommodation for illegal immigrants currently in the United States.

"We cannot neglect our economic security and that is exactly what we're doing by neglecting comprehensive immigration reform," said Gutierrez, who worked with Chertoff and a bipartisan group of lawmakers in support of the failed 2007 immigration legislation.

E-Verify is used by slightly more than 69,000 companies, with about 1,000 additional companies signing up to join the free, Internet-based system each week.

Many companies have enrolled because of the administration's stepped-up immigration raids. In industries that traditionally rely on immigrant labor, such as meatpacking, company managers understand that not using E-Verify can prompt immigration officials to take a closer look.

Chertoff said E-Verify clears 99.5 percent of qualified employees automatically. But a 2006 report by the Social Security's inspector general found discrepancies in 17.8 million records for citizens and legal immigrants that would create a "significant workload" to correct.

Lawmakers and other critics warned that forcing the more than 200,000 federal contractors to join E-Verify could overwhelm the Social Security Administration and create havoc for legal workers.

"As the administration requires more employers and workers into E-Verify, it should at the same time ensure that the system does not impinge upon U.S. citizens' fundamental right to earn a living," said Rep. Zoe Lofgren, D-Calif., chairwoman of the House subcommittee on immigration.

Companies doing business with the government risk losing their contracts if they break federal rules. Among business executives, some worry that the new requirement could add to expenses.

"There's concern about increased costs and delays in hiring brought about by inaccuracies in the database," said Neal Couture, executive director of the National Contract Managers Association, a professional group.

Tim Sparapani of the American Civil Liberties Union claimed that E-Verify is "not real immigration enforcement" because the system cannot detect applicants who use documents stolen from a legal worker. He predicted that the system would prompt more identity theft by illegal immigrants.

"American workers' identities are essentially going to become a black-market commodity," Sparapani said.

Still, many people were pleased by Monday's action. "With today's announcement by Secretary Chertoff, we are diminishing the ability of illegal immigrants to find employment in the United States," said Rep. Brian Bilbray, R-Calif.

Added Dan Stein of the Federation for American Immigration Reform, which advocates restricting both legal and illegal immigration: "It's an excellent idea, long overdue."