NEW YORK, AUG. 13 -- A federal judge today barred enforcement of the Helms amendment, passed hastily by Congress last year in an attempt to prevent minors from listening to sexually explicit telephone recordings.

Saying the law "presents a threat of imminent irreparable harm to First Amendment freedoms," U.S. District Judge Robert P. Patterson Jr. ruled two days before the sweeping law was scheduled to take effect.

The law, aimed almost exclusively at the lucrative and fast-growing "dial-a-porn" industry, prohibits obscene telephone messages, bans any arrangement that makes indecent -- but not obscene -- messages available to persons under 18 and requires adults who want to hear them to subscribe to the services in writing.

Although Patterson's ruling affects only New York state, Federal Communications Commission (FCC) officials said it could be adopted nationally.

"The judge did a courageous and important thing today," said Joel Dichter, who represented five New York dial-a-porn companies in the suit in Manhattan federal court. "The law would have required customers to make a written request to obtain their First Amendment rights. That's ridiculous. There are far better ways to protect minors from obscenity."

The ruling is the first by a federal judge that confronts the emotional issue in which First Amendment rights appear to conflict with compelling problems faced by parents seeking to protect their children from access to inappropriate telephone messages.

Assistant U.S. Attorney Nancy Kilson said the ruling would be reviewed before a decision on whether to appeal.

Many people, particularly parents of young children, have become appalled at the simplicity with which anyone who can use a phone can gain access to extremely explicit services. But Patterson ruled there are better ways to restrict access than by political fiat or censorship.

The amendment sponsored by Sen. Jesse Helms (R-N.C.) "emerged from the Senate in the waning days of a congressional session," Patterson wrote in granting the preliminary injunction. "No committee reviewed the bill, and the Senate only had a few minutes to consider the proposed bill."

In addition to First Amendment issues, the judge wrote that the law was too broad and vague and that the government had ignored potentially useful and less restrictive means of screening calls.

In New York, the District, Maryland and many other localities, for example, a subscriber can simply call the telephone company and request that access be blocked to such pay calls by technologically screening the "700" and "900" numbers used by such services.

Last year, the FCC, which monitors and enforces laws applicable to telephone companies, decided that voluntary blocking would not be sufficient to protect minors from indecent messages.

Telephone pornography has become a multimillion-dollar business, with sales increasing rapidly, according to companies providing the services. The New York Telephone Co. has projected that implementation of the Helms amendment would result in a reduction of more than 7 million pay calls each year in New York state alone.

"This is a significant decision," said Jane Mago, assistant general counsel at the FCC, although she said she had not yet read it fully. "This is the first time there has been a real review on the issues. But it's too soon to know how widely it will be applied."

Dial-a-porn services, for which callers pay a premium to hear sexually explicit messages, have been growing rapidly since their introduction. Opponents of the services say that they do not deserve the free-speech protection and that they endanger children's health.

Last year, the Supreme Court struck down a different law aimed at telephone sex services, saying it was too broad. The high court has issued guidelines to help local judges and legislators to determine whether something is obscene, but it has never issued such a ruling on how to define indecent expression.

Companies providing commercial-sex messages by phone have said requiring subscriptions in writing would destroy their business and unfairly restrict an individual's rights. The law would require telephone companies to block any such service to households that have not requested it in writing.