Federal immigration authorities can revoke an immigrant's naturalized citizenship if it was obtained by fraud, a federal appeals court ruled today.

The 2 to 1 ruling by a panel of the 9th U.S. Circuit Court of Appeals overturned the decision of a Seattle judge who said the power of the Immigration and Naturalization Service to revoke citizenship for fraud was open to serious question.

The case arose from a class action lawsuit that challenged the INS's authority to revoke the citizenship of immigrants who failed to disclose their past arrests. The immigrants argued that only a court could revoke their citizenship.

A 1990 law authorized the INS to grant citizenship to qualified immigrants, a power that was previously handled by the courts. But the INS said the power to grant citizenship included the power to revoke it as well, although that was not expressly stated in the wording of the statute.

Today, the appellate panel said the power to revoke was implicit in the power to naturalize.

The INS adopted the rules in 1996 after the agency conducted a study of the 1 million persons granted citizenship between August 1995 and September 1996. The study found that about 6,300 persons allegedly lied or failed to inform the INS about prior arrests. However, the agency acknowledged that only 369 of those people had committed crimes that would have made them ineligible for citizenship.

Proponents of the new policy said that lying under oath -- as most of those targeted under the new program were accused of doing -- should disqualify applicants from the privileges of citizenship.

But immigrant advocates said that the new program violated their clients' constitutional right not to testify against themselves and in numerous instances unfairly targeted people for innocent or minor mistakes.

The INS had instituted 2,722 denaturalization proceedings under the program and had revoked the citizenship of 16 people at the time U.S. District Judge Barbara J. Rothstein issued her injunction. The agency had another 1,900 cases under review at that time. Those can now go forward.

Jonathan S. Franklin, a lawyer for the immigrants, said he would appeal by asking for a hearing before the full appeals court. The case affects about 4,500 people.

"Citizenship is among a person's most cherished rights," he said. "This is the first time any court has ever held that the INS may take that right away on its own, without going before a judge."

INS spokesman Bill Strassberger said the agency was pleased with the decision. "We feel that the administrative denaturalization process is less cumbersome than the existing judicial denaturalization process."