Thousands of sex offenders who were required to register with the state for a decade after leaving prison will now register for life.

A decision yesterday by a federal appellate court will likely bring an end to a decade-long legal dispute over the state’s authority to register criminals whose sex crimes date back to before the creation of the state sex offender registry in 1996. In 2004, several thousand of the offenders settled with state lawyers and agreed to be registered until 2006, which was 10 years after the creation of the registry. The 4,400 sex offenders involved in the settlement had committed crimes that were judged not to be the most heinous.

Subsequently, the Legislature sought to thwart that settlement by passing a law whose effect was to require many of those sex offenders to register for life. The new law also gave law enforcement more authority to inform the community about an individual’s crimes in some instances.

In a 2–1 decision yesterday, a panel for the U.S. 2nd Circuit Court of Appeals threw out the separate agreement between the sex offenders and the state. The decision says the recent amendments by the Legislature apply to the 4,400 sex offenders, regardless of the 2004 agreement. The court’s ruling reverses a decision last year by a trial judge, who upheld the agreement.

Yesterday, the dissenting judge, Rosemary Pooler, wrote that the majority had “ignored basic principles of contract law” in throwing out the agreement.

A Legal Aid attorney for the plaintiff, Thomas O’Brien, said yesterday’s decision allowed the state “to basically renege” on the earlier court settlement.

“The 10 years was basically like the finish line, and the state has hosed it away,” Mr. O’Brien said. “Now the 2nd Circuit said that’s okay.”

The majority decision, written by Judge Jon Newman and signed by Jose Cabranes, holds that the settlement “cannot be interpreted to preclude the application of subsequent legislative changes.”

The likely immediate effect of the decision will be that thousands of the offenders will ask for hearings to be reclassified as lesser offenders, Mr. O’Brien said. Of the 4,400 plaintiffs, about 3,000 are classified as level two offenders, while the rest are level one offenders, which include less serious crimes.