NEW YORK, AUG. 27 -- Two men charged with operating an unlicensed radio station from a ship will not be prosecuted if they promise not to resume the illegal broadcasts, federal officials said today.

But on hearing the good news, Ivan Rothstein and Alan Weiner, known as the "radio pirates," vowed to resume broadcasting as soon as possible.

And on hearing the pirates' defiant response, Assistant U.S. Attorney Matthew Fishbein in Brooklyn said, "If they are dumb enough to resume broadcasting, then we would prosecute -- not only for future violations but for past violations."

Rothstein and Weiner were arrested July 28 by the Coast Guard and agents of the Federal Communications Commission in a predawn raid. Their ship, Sarah, served as a studio for their radio station RNI -- Radio New York International -- which broadcast "free-form" rock and roll.

Rothstein, 25, and Weiner, 34, were charged with breaking international radio law and faced a possible five-year prison term and $250,000 fine.

Andrew J. Maloney, the U.S. Attorney for the Eastern District of New York, said today that "no further governmental purpose would be served by pursuing the criminal charges. By shutting down the illegal station, the FCC achieved what it set out to accomplish."

Maloney noted that because of the intricacies of international radio law, there was a legitimate question whether the defendants understood that their broadcasts were illegal.

Rothstein and Weiner repeatedly had said they were within their First Amendment rights and maintained today that they do not need an FCC license. The "pirates" had anchored their floating radio station four miles off the Long Island coast and claimed they were in international waters.

The government chose not to prosecute, Fishbein said, because "they are not drug dealers or career criminals. If they abide by the law from now on, in the interest of justice, we thought this was the right thing to do." But he warned, "We have clearly established that what they have done is against the law. Unofficially, they're on probation. If they resume broadcasting, at that point we have given them every opportunity and will prosecute."

Margaret Mayo, a lawyer representing the "pirates," challenged the government's threat against her clients. "What's the point of dropping the case if they had done something illegal?" Mayo asked. She said she is considering filing a $1 million lawsuit against the Coast Guard and the FCC for false arrest and civil rights violations.

The "pirates" say they plan to regain their rusty 200-foot vessel, which sprang a leak after the Coast Guard raid and is being repaired at a Boston shipyard. Their broadcasting equipment also needs repair since the Coast Guard and the FCC dismantled it.