Philadelphia ? Software filters work much better than a 1998 federal law designed to keep pornography away from children on the Internet, a federal judge ruled Thursday in striking down the measure on free-speech grounds.

Senior U.S. District Judge Lowell Reed Jr. also said the Child Online Protection Act fails to address threats that have emerged since the law was written, including online predators on social-networking sites like MySpace, because it targets only commercial Web publishers.

“Even defendant’s own study shows that all but the worst performing (software) filters are far more effective than COPA would be at protecting children from sexually explicit material on the Web,” said Reed, who presided over a monthlong trial in the fall.

The never-enforced law was Congress’ second attempt to protect children from online porn. The U.S. Supreme Court upheld in 2004 a temporary injunction blocking the law from taking effect; Reed on Thursday issued a permanent injunction.

The law would have criminalized Web sites that allow children to access material deemed “harmful to minors” by “contemporary community standards.” The sites would have been expected to require a credit card number or other proof of age. Penalties include a $50,000 fine and up to six months in prison.

Sexual health sites, the online magazine Salon.com and other Web sites backed by the American Civil Liberties Union had challenged the law on grounds it would have a chilling effect on speech. Joan Walsh, Salon.com’s editor in chief, said the law could have allowed any of the 93 U.S. attorneys to prosecute the site over photos of naked prisoners at Iraq’s Abu Ghraib prison.

“The burden would have been on us to prove that they weren’t” harmful to minors, Walsh said Thursday.

In his ruling, Reed warned that “perhaps we do the minors of this country harm if First Amendment protections, which they will with age inherit fully, are chipped away in the name of their protection.”