Prison authorities must now get a judge’s approval if they wish to commit sex offenders to psychiatric hospitals in anticipation of their release from prison.

A unanimous decision by the Court of Appeals yesterday said state authorities acted illegally last year when they followed Governor Pataki’s instructions to transfer violent sex offenders to psychiatric hospitals to prevent them from walking out of prison when their sentences ended. Mr. Pataki began using the state’s Mental Hygiene Law, which was not intended for inmates, for that purpose after he failed last year to push through the Legislature a law that would call for civil commitment of sex offenders after prison time.

The court’s decision, which expressed sympathy for Mr. Pataki’s goal, did not order the immediate release of any sex offenders who were committed under Mr. Pataki’s policy. The 12 anonymous sex offenders who brought the lawsuit are however entitled to immediate court hearings to determine whether their continued detention is warranted, according to the decision. The ruling says nothing about the other 100 sex offenders who are currently committed in mental institutions pursuant to Mr. Pataki’s instructions.

The decision instructs prison wardens on how to proceed when they want to transfer sex offenders to psychiatric institutions. Inmates have the right to appear before a judge prior to being involuntarily committed to a psychiatric institution, according to the decision. Over the past year, all that was required was the opinion of three state-employed psychiatrists that an inmate should not be released to approve such a transfer, according to the decision.

Attorney General Eliot Spitzer, whose office defended and lost the case, will push for a civil commitment law when he becomes governor, a spokeswoman for Mr. Spitzer, Christine Anderson, said in a statement sent via e-mail. But he may not get the chance if Mr. Pataki succeeds first in passing such a law through a special legislative session, as he stated yesterday that he would try to do.

“In light of today’s decision and considering the danger it poses to the most vulnerable of New Yorkers, in the coming days I will call a special session to consider legislation to address this critical issue,” Mr. Pataki said in a statement sent via e-mail.

At Mr. Pataki’s instructions, mental health and correctional officers have screened 787 incarcerated sex offenders for forcible commitment prior to their prison release. Of those inmates, 120 were confined following psychiatric testing by the state’s Office of Mental Health, a spokesman for Mr. Pataki, David Catalfamo, said. Since then, at least one of the sex offenders died while as many as seven others have been released Mr. Catalfamo said.

The court expressed some sympathy with Mr. Pataki’s efforts. “We understand how in an attempt to protect the community from violent sexual predators, the State proceeded” to use civil law to commit inmates to the institutions, the decision says.

“We do not propose that these petitioners be released, nor do we propose to trump the interests of public safety,” the decision, written by Judge Carmen Beauchamp Ciparick, said. “Rather, we recognize that a need for continued hospitalization may well exist.”

The U.S. Supreme Court has repeatedly upheld the constitutionality of state laws committing sex offenders to psychiatric institutions after their prison terms expired. In New York, where no such explicit law exists, the legal issue before the Court of Appeals was whether state authorities could use the existing Mental Hygiene Law. That law, not previously used in the penal system, is intended to allow for the involuntarily hospitalization of mentally ill members of the public at large, not prison inmates. The law does not require a judge’s order for involuntary commitment, but only the determination of three physicians that the person is mentally ill, requires care, and is a danger to self or others.

The court found that the state was wrong to sidestep the legal protections that are afforded inmates under state corrections law, which allows an inmate to go before a judge and challenge any attempt to be transferred to a mental hospital.

“A strong argument can be made that the petitioners were constitutionally entitled to a hearing before being deprived of the liberty that they would otherwise have obtained upon the completion of their prison terms,” Judge Robert Smith wrote in a concurring opinion.

One expert on mental health and the law lauded the decision for telling prison officials the state’s Mental Hygiene Law was off-limits to prison wardens.

“Generally, I think there has been an inappropriate blurring between the penal correctional interest and the mental health and rehabilitation interest,” Michael Perlin, a professor at New York Law School, said. “This decision seems to be righting the balance a little.”

The effect of the decision could turn out to be minor, one lawyer involved in the case said.

“It’s really just a procedural decision,” a director of the Mental Hygiene Legal Services for an upstate judicial district, Emmett Creahan, said. “It’s just saying you have to offer them a hearing before they go to the hospital.”

The decision reverses a state appellate court ruling that said the plaintiffs, who were all inmates within hours or days of their release, were no longer inmates for the purposes of the law. The Court of Appeals decision affirms the decision of a state Supreme Court judge, Jacqueline Silberman.