PHILADELPHIA, June 23 (AP)--A federal law aimed at protecting children from online pornography will probably be ruled unconstitutional, an appeals court said in upholding an injunction against the measure. The Child Online Protection Act requires commercial Web sites to collect a credit card number or some other access code as proof of age before allowing Internet users to view online material deemed harmful to minors. The 3rd U.S. Circuit Court of Appeals said Thursday the proof of age requirement places an undue economic burden on publishers, who would have to pay for a screening system and could lose users who did not want to register. It also said the act's definition of harmful material as that which offends "contemporary community standards" is impossible to enforce because community standards vary, and Internet publishers do not know where their users live. "'Harmful to minors' means one thing in New York and another in a small southern community," said David Sobel, lawyer for the Electronic Privacy Information Center, one organization challenging the law. "I think this decision creates a real challenge to the advocates of Internet censorship. I don't see a way that any censorship can survive given this decision." The court said the law is not the least restrictive way to protect children from harmful material, noting that parents can install blocking software on their own computers. Senior U.S. Circuit Judge Leonard I. Garth called the law ``Congress' laudatory attempt to achieve its compelling objective of protecting minors from harmful material on the World Wide Web,'' but said in the court's 34-page ruling that it will probably be ruled unconstitutional. Other plaintiffs in the suit against the law include the American Civil Liberties Union, the online publication Salon, the Philadelphia Gay News, bookstores and sexual health publishers. A Justice Department spokeswoman said it was too early to say whether there would be an appeal. Garth said Congress had not adequately addressed concerns that led the Supreme Court to strike down an Internet censorship law passed in 1996. The act currently under debate, passed in 1998, is limited to commercial sites and defines harmful material as that which would offend "contemporary community standards." One of the bill's sponsors, Rep. James Greenwood, R-Pa., said Thursday that if attorneys decide more legal action on the current law would be fruitless, he believes Congress will pass another version of the bill in an attempt to meet the constitutional objections. Greenwood said he believes the intent of the bill - to keep children out of pornographic sites in the same way they are barred from adult bookstores and X-rated movie theaters - is constitutional, although there are technological difficulties in doing so. "So far, what disturbs me is that the courts are erring on the side of allowing the children in rather than making it more difficult for adults," Greenwood said.