"We hold that such a complaint should be dismissed," Justice David Souter wrote for the court majority.

At issue is a class action lawsuit against Bell Atlantic, which contends that lack of competition for local telephone service and "parallel conduct" of the regional Bells were evidence of an antitrust conspiracy.

More than a decade after the court-ordered breakup of AT&T into seven regional Bells, Congress passed a landmark legislation in 1996 designed to foster competition among carriers. It required regional Bells such as Verizon to make their networks available to rivals in exchange for gaining access to the lucrative long-distance voice and data markets.

The case was dismissed by a district court judge, who said the plaintiffs could not sue based only on a "bare-bones" allegation of conspiracy.

But that decision was reversed by a federal appeals court in New York, which ruled the case should not be dismissed unless the charges were implausible.

The U.S. Justice Department's antitrust division has sided with the telephone companies, arguing that "parallel action and inaction" alone was not enough to provide a basis for an antitrust lawsuit.

The Supreme Court's majority opinion agreed, ruling that an allegation of parallel conduct and a bare assertion of conspiracy is not enough. There must be enough facts to suggest an agreement was made, Souter said.