"The Court expresses no view as to whether COPA suffers from substantial overbreadth for reasons other than its use of community standards, whether the statute is unconstitutionally vague, or whether the statute survives strict scrutiny," said Justice Clarence Thomas, writing for the majority.

The court vacated the lower court's decision, and remanded the case back to the appeals court, saying "prudence dictates allowing the Third Circuit to first examine these difficult issues."

Today's action, however, has no effect on the 1999 district court ruling that bars the government from enforcing the law.

Passed by Congress after a previous law - the Communications Decency Act (CDA) - was ruled largely unconstitutional by the Supreme Court, COPA also became mired in murky constitutional waters shortly after it was signed into law by former President Clinton.

ACLU Senior Staff Counsel Chris Hansen said the court's refusal to overturn the enforcement restrictions was the most telling part of the decision.

"To my mind, there remains a significant sentiment on the court that the statute may be unconstitutional in some form or another," he said. "The fact that they went out of their way to keep the restrictions in place is significant."

The Justice Department said that the law merely reflects practices in the physical world.

"Congress carefully drafted the law to put Internet pornography on the same footing as material offered for sale in bricks-and-mortar bookstores or convenience stores, where children are protected from inadvertent viewing because magazines are wrapped in brown paper or are kept behind the counters," said Justice Department spokeswoman Barbara Comstock in a statement.

Center for Democracy and Technology Associate Director Alan Davidson said today's decision does not represent a serious setback for COPA's opponents.

"We fully expect this case to end up back before the Supreme Court on a different set of issues," Davidson said. "Nothing in this opinion changes our belief that this statute will be ultimately found unconstitutional just as the Communications Decency Act was before it."

By choosing to focus solely on the question of whether a community standards test is valid in an online environment, the court failed to even address some of the most compelling arguments against COPA, Davidson said.

The decision leaves civil liberties advocates free to challenge COPA on the grounds that it unfairly restricts adults from viewing constitutionally protected material on the Internet, Davidson said.