A judge ruled yesterday that the state Department of Motor Vehicles cannot proceed with a plan to revoke hundreds of thousands of driver’s licenses based on immigration status.

The DMV lacks the expertise and the statutory authority to act as “an enforcer” for the Department of Homeland Security, a state Supreme Court judge, Karen Smith, wrote in a decision released yesterday.

The decision came the same day that federal legislation barring states from giving driver’s licenses to illegal immigrants received final approval in Congress. The bill that included the driver’s-license measure passed the Senate on a 100-0 vote. It now goes to President Bush, who supports it.

In Manhattan, Justice Smith ruled that the state’s pending suspension of 252,000 licenses, many of which were gained with fraudulent Social Security numbers, is unlawful. The DMV responded that it intends to appeal, which places a stay on the ruling so that undocumented immigrants cannot renew licenses despite the ruling.

In 2002, as part of an attempt to tighten security following the September 11 attacks, the agency began to check its records against a national database of Social Security numbers. Of 11.5 million New York licenses, 600,000 were found to have been obtained with repeat or nonexistent numbers. The DMV sent letters to those drivers threatening suspension if no action was taken. Before Justice Smith issued an initial restraining order in February, 7,000 licenses were suspended.

“By meeting our statutory obligation to ensure that applicants for driver’s licenses are who they say they are, New York State is fighting fraud and protecting New Yorkers from terrorism and other criminal acts. Verifying a person’s Social Security number is a common-sense and proven way to help fulfill that important responsibility,” a spokesman for the DMV, Joe Picchi, said in a written statement. “New York State’s driver’s license is among the most secure in the nation, and we will continue to enforce our laws to ensure that it remains that way.”

The judge, however, ruled the DMV lacks the authority to take those actions, which she said could be enacted only by a legislature. The statutory requirements to obtain a license are proof of 1) identity, 2) age, 3) fitness, and 4) a Social Security number or ineligibility for a Social Security number. According to state law, an immigrant who does not have a Social Security number can receive a letter from the Social Security Administration saying he is ineligible to receive one. That letter, with other identity documents, can be used to receive a license. Justice Smith wrote that the DMV must accept these letters.

“DMV cannot make current immigration documents, or any other documentary proof of one’s immigration status, the only documents that are acceptable to verify an applicant’s identity,” she said. New York is one of 12 states that currently do not grant licenses to undocumented immigrants.

Justice Smith also rejected the state’s “temporary visitor” program begun in 2003, which provided temporary licenses with expiration dates based on a person’s immigration status.

The Puerto Rican Legal Defense and Education Fund, which represents the five plaintiffs in the suit, said the ruling would benefit immigrants and send a warning to states, which will need to rewrite requirements for driver’s licenses as part of the federal homeland security measure.

Other legal experts said that, regardless of the ruling, the debate over whether the New York DMV can require a Social Security card for a license will probably soon be moot.

The legislation Mr. Bush is expected to sign this week would impose national standards for driver’s-license applications. It was part of an $82 billion “emergency spending” package to provide added financing for military operations in Iraq and Afghanistan.

The Real ID Act, as proponents have called it, would require multiple identification documents before states could issue ID cards besides prohibiting issuance of licenses to illegal immigrants.

Backers of the measure, whose lead sponsor is Rep. James Sensenbrenner, a Republican of Wisconsin, said it provides for security lapses that the 9/11 commission identified in pushing for stricter guidelines for driver’s licenses.

In contrast, a staff attorney for the Puerto Rican legal fund, Jackie Chin, said that when illegal immigrants do not have licenses, the state becomes less safe, because government no longer has data on hundreds of thousands of people. The suit’s advocates also have said the DMV’s policies would lead to the proliferation of fraudulent licenses.

The offices of Senators Schumer and Clinton told The New York Sun that the two New York Democrats were sorry the Real ID Act was attached to the emergency appropriations bill and not considered separately by Congress.