Philadelphia ? A three-judge panel Friday struck down Congress’ third and latest attempt to shield children from Internet porn, ruling that public libraries cannot be forced to install software that blocks sexually explicit Web sites.

The federal panel unanimously found that the Children’s Internet Protection Act relies on filtering programs that also block sites on politics, health, science and other topics that should not be suppressed.

Internet users work at the computers at the Philadelphia Public Library. Three federal judges Friday threw out a federal law that would have forced public libraries to equip computers with software designed to block access to Internet pornography.

“Given the crudeness of filtering technology, any technology protection measure mandated by CIPA will necessarily block access to a substantial amount of speech whose suppression serves no legitimate government interest,” the judges wrote.

Three times since 1996, Congress has enacted laws aimed at keeping youngsters from seeing Internet porn. And all three have been struck down.

The latest law, signed by President Clinton in 2000, was supposed to go into effect July 1. It would require public libraries receiving federal technology funds to install the filters on their computers or risk losing that aid. Schools and school libraries are still subject to the law.

Barbara Comstock, spokeswoman for Atty. Gen. John Ashcroft, said the Justice Department was disappointed and may appeal to the Supreme Court.

Conservatives said the ruling tied the hands of parents trying to protect children. “These groups are more concerned with providing access to smut than they are protecting child patrons and employees,” said Ken Connor, president of the Family Research Council.

The ruling was welcomed by the American Library Assn. and the American Civil Liberties Union, which had argued that the law would make it tougher for people without home computers to get information on topics such as breast cancer and homosexuality, which are sometimes accidentally blocked by the filters.

“It is certainly my hope that now that Congress has taken three strikes, it will get out of the business,” said Stefan Presser, the ACLU’s legal director in Pennsylvania.

Echoing earlier court decisions, the three-judge panel wrote that the Internet is an open public forum and that any move to exclude certain content must be narrowly tailored.

It said there were less restrictive ways than software filtering of shielding children from Internet porn. Those options included requiring parental consent before a minor is allowed to use an unfiltered computer, or requiring a parent to be present while a child surfs the Internet.

The law would have allowed adults to ask that the filtering technology be turned off. But the court held that some library patrons might be embarrassed by having to ask, and some librarians may not have the technical expertise to comply with such a request.

Justice Department lawyers had argued that Internet smut is so pervasive that protections are necessary to keep it away from youngsters, and that the law simply calls for libraries to use the same care in selecting online content that they use for books and magazines.