The Social Security Administration, cracking down on the use of Social Security numbers in the aftermath of Sept. 11, no longer will issue numbers to foreigners who apply for driver's licenses even in states that require them.

The agency also has stepped up efforts to identify mismatches between names and Social Security numbers with help from employers nationwide, sometimes revealing immigrants working in the United States illegally.

The changes have been in the works for some time. They were given impetus by the Sept. 11 terror attacks, in which some of the 19 hijackers falsely obtained Social Security numbers that allowed them to operate more freely in the country by opening bank accounts and getting credit cards.

One union that represents a large number of immigrant workers cautioned that the post-Sept. 11 crackdown on foreigners is building a lot of resentment and anger.

``People feel like they're under attack for no good reason other than somebody has decided this makes good headlines,'' said Eliseo Medina, an executive vice president of the Service Employees International Union, which has 1.5 million members. ``All of these activities really don't do anything to advance security. What they're going to do is create huge problems for workers and employers equally.''

Federal sweeps by law enforcement officials also have netted hundreds of airport workers across the country accused of falsifying Social Security applications and violating immigration laws.

The stricter rules for issuing Social Security numbers will not affect foreigners who need them to work or file for federal benefits. Last year, Social Security issued more than 18.4 million cards, and about 1.5 million went to foreigners.

But the numbers will no longer go to foreigners to apply for driver's licenses or to register vehicles.

The change ``is intended to protect the integrity of the Social Security number,'' said Carolyn Cheezum, spokeswoman for the Social Security Administration. ``This policy is consistent with the original intent of the number, which is to keep an accurate record of earnings for future benefits and to monitor benefits paid.''

The number instead has become in many ways a national identifier, getting used for all kinds of things, such as gym memberships, ID cards, medical insurance and education records.

Only 10 states required Social Security numbers to obtain driver's licenses when the administration made the change, Cheezum said. They were Alabama, Hawaii, Illinois, Iowa, Kentucky, Mississippi, New Mexico, Pennsylvania, Utah and West Virginia.

It's up to those states to deal with foreigners who are required to have a Social Security number to obtain a license or register a car but are refused one because of the administration's new rules, Cheezum said. The agency does not track how many people might be covered.

The Social Security Administration also is intensifying its investigation of mismatched names and Social Security numbers. It expects to send out 750,000 letters — compared with 110,000 last year — to employers and workers this year, notifying them that a name or number on a wage report does not match the agency's records.

Sometimes the mismatch is a typographical error or other mistake, or it could reveal that a worker is in the country illegally. ``Instead of waiting to be terminated, they quit and move on,'' the service union's Medina said.

The union has tried to help such workers, employed in janitorial or other low-paying service jobs, in cities such as Portland, Ore., Los Angeles and Chicago.

The Social Security Administration previously sent letters only if 11 or more mismatches were seen at a company or if mismatches represented 10 percent or more of the company's work force.

Now, one mismatch at a company prompts a notification letter.

The agency says it is trying to improve accuracy of wage reporting so that workers will get the correct monthly Social Security benefits.

The Social Security agency does not have access to electronic information from the Immigration and Naturalization Service and cannot investigate before issuing Social Security numbers to foreigners.

A report issued this month by James Huse Jr., inspector general of the Social Security Administration, said the agency is looking at ways to verify better information it receives from applicants, but because of problems of coordinating with the immigration service, a fix remains six months or more away.

Huse said the administration fears that thousands more improperly assigned numbers will be issued because of the delay.