WASHINGTON - Internet sites that offer sexually explicit material may soon face their first real threat of criminal prosecution, judging from the friendly reception the Supreme Court gave Tuesday to a disputed and never-enforced federal law. The Child Online Protection Act would impose fines and jail terms on those who post explicit text or images on a commercial Web site that can be tapped by minors. Though passed by Congress in 1998, it has been blocked by federal judges who cited free-speech concerns.

But the Supreme Court took up the government's appeal Tuesday, and several justices said it made sense to create a type of barrier on the Internet that would prevent children from having access to pornography. To comply with the law, for example, pornographic Web sites might require viewers to give credit card numbers or other evidence that the users are adults.

These and other requirements would re-create the restrictions on pornography that are familiar to earlier generations, supporters of the federal law say. Where adults were free to enter X-rated theaters or bookstores, minors were not. And retail stores typically have opaque racks or other means of preventing minors from perusing sexually oriented magazines.

The Internet has broken down those barriers and given minors free access to the most explicit pornography, U.S. Solicitor General Theodore B. Olson told the court. Type in the words "free porn" on an Internet search engine, he said, and you get a list of 6 million sites. "They are all available to you or to me or to a 12-year-old," he told the justices.

The federal law would force Web sites to create a modern "counterpart to the blinder rack," Olson said, referring to the opaque store racks that hide sex magazines.

According to a survey done for Congress, the government said, 70 million persons visit pornographic Web sites each week, and 16 percent of the visitors - or about 11 million persons - are under age 18.