PHILADELPHIA – A Pennsylvania law requiring Internet service providers to stop customers from viewing Web sites containing child pornography has been a failure and should be declared unconstitutional, a lawyer for a civil liberties group told a federal judge last week.

Legislators were ill-informed about how the Internet worked when they crafted a 2002 law giving the state’s attorney general the power to order companies like America Online and Verizon to block access to Web sites with illegal content, said Center for Technology & Democracy lawyer John Morris in the last day of oral arguments in the case.

Much as phone companies can’t control what people fax over phone lines, Internet service providers can’t control material on the Web, opponents of clampdown measures say. Efforts to use sophisticated filters to stop people from seeing illicit sites, they contend, have also proven problematic.

In two years, Morris claimed, ISPs trying to comply with orders to block 411 Web sites have been forced to temporarily sever access to at least 1.5 million legal Web pages with no connection to child porn. About 500,000 of those sites remain off-limits to customers of certain ISPs today, he said.

Morris called the law “well-intentioned, but fundamentally flawed.”

John Shellenberger, a senior deputy attorney general, told U.S. District Judge Jan E. DuBois that the state’s request that ISPs stop customers from accessing known child-porn sites was both reasonable and technologically possible. He argued that the ISPs primarily made business decisions when they blocked thousands of sites rather than buy or develop more refined blocking systems.

Experts have said the technology exists, but that ISPs don’t have it or can’t afford it. Many also question how long it would be effective against the ever-changing tactics of those posting kiddie porn on the Internet.

Shellenberger also said the case illustrated the difficulty legislatures have had with trying to stop the flood of “vile material” over the Net.

The Supreme Court struck down the 1996 Communications Decency Act, which made it a crime to put sexually explicit material online where children could find it. The Court ruled in Reno v. ACLU (1997) that the law violated free-speech rights in.

In 2002 in Ashcroft v. ACLU the Court barred enforcement of the 1998 Child Online Protection Act, which required Web sites to collect credit card numbers or other proof of age before letting Internet users view material deemed harmful to minors.

The Supreme Court finally gave Congress a victory last year in United States v. American Library Association it upheld a law requiring public libraries to equip computers with anti-pornography filters as a condition of receiving federal funding.

Civil liberties groups had challenged the law requiring the filters, arguing that they were clumsy and had also unintentionally blocked access to a significant amount of legal content.

That case differed from Pennsylvania’s Internet blocking efforts because installing the filters was, to some extent, voluntary. Libraries not wishing to have the software could simply reject federal aid.

Lawyers for the Center for Technology & Democracy and the American Civil Liberties Union have argued that Pennsylvania’s attempts to regulate the Internet are improper because the Constitution gives sole power to regulate interstate commerce to the federal government.

DuBois has been considering evidence and taking testimony in the case since January. His decision could take additional weeks or months.