An American judge has struck down a law passed in 1998 by the US Government that made it a crime for commercial website operators to let children access “harmful” material.

The judge said parents can protect their children through software filters and other less restrictive means that do not limit the rights of others to free speech.

“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,” wrote Senior US District Judge Lowell Reed Jr., who presided over a four-week trial last fall.

The law would have required websites deemed harmful to children by “contemporary community standards” to ask for a credit card number or other proof of age before granting access. Penalties included a $50,000 fine and up to six months in prison.

Sexual health sites, the online magazine Salon.com and other websites backed by the American Civil Liberties Union challenged the law. They argued that the Child Online Protection Act was unconstitutionally vague and would have had a chilling effect on speech.

The US Supreme Court upheld a temporary injunction in 2004 on grounds the law was likely to be struck down and was perhaps outdated.

Technology experts said that parents now have more serious concerns than websites with pornography. For instance, the threat of online predators has caused worries among parents whose children use social-networking sites such as MySpace, which is owned by News Corp, parent company of Times Online.

The case sparked a legal furore last year when Google challenged a Justice Department subpoena seeking information on what people search for online. Government lawyers had asked Google to turn over a million random web addresses and a week’s worth of Google search queries.

A judge sharply limited the scope of the subpoena, which Google had fought on trade secret, not privacy, grounds.

To defend the nine-year-old Child Online Protection Act, Government lawyers attacked software filters as burdensome and ineffective, even though they have previously defended their use in public schools and libraries.

“It is not reasonable for the Government to expect all parents to shoulder the burden to cut off every possible source of adult content for their children, rather than the government’s addressing the problem at its source,” a government lawyer, Peter D. Keisler, argued in a post-trial brief.

Critics of the law argued that filters work best because they let parents set limits based on their own values and their child’s age.

The law addressed material accessed by children under 17, but applied only to content hosted in the United States.

The websites that challenged the law said fear of prosecution might lead them to shut down or move their operations offshore, beyond the reach of the US law. They also said the Justice Department could do more to enforce obscenity laws already on the books.

The 1998 law followed Congress’ unsuccessful 1996 effort to ban online pornography. The Supreme Court in 1997 deemed key portions of that law unconstitutional because it was too vague and trampled on adults’ rights.

The newer law narrowed the restrictions to commercial websites and defined indecency more specifically.

In 2000, Congress passed a law requiring schools and libraries to use software filters if they received certain federal funds. The high court upheld that law in 2003.