Court Upholds F.C.C. Rule on Use of Data Networks WASHINGTON - Cellphone companies must allow customers of competing wireless carriers to use their networks for the Internet and e-mail when outside their home territory, a federal appeals court said here on Tuesday. The United States Court of Appeals for the District of Columbia said that just as the Federal Communications Commission required wireless carriers to allow voice-service roaming by customers of other carriers, it also can require the same, at commercially reasonable rates, for data customers - in essence, those who use smartphones, tablets and other wireless devices. The judges" 3-to-0 decision is a significant victory for the F.C.C. The agency has lost authority over Internet communications in recent years. And it is a big win for smaller cellphone companies, who now have the leeway to offer customers national calling and data plans, albeit ones that could generate extra charges. On the losing side is Verizon Wireless, which had challenged the agency's data roaming order in 2011. The company argued that the agency did not have authority to oversee data communications on wireless broadband networks and that it was imposing "common carrier" regulations on companies - essentially, regulating them like public utilities, as it does with home phone service. Verizon said that it already had data roaming agreements so there was no need to codify the practice. "As we made clear throughout the case," Ed McFadden, a company spokesman, said on Tuesday, "Verizon Wireless regularly enters into such data roaming agreements on commercially reasonable terms to meet the needs of consumers, and will continue to do so." The decision has broad implications for the agency, analysts said. "This does bode well for the F.C.C."s ability to assert its authority in regulating wireless services," said Andrew Jay Schwartzman, senior vice president and policy director for the Media Access Project, a nonprofit law firm that promotes consumer choice. "This is the first time these issues have come up in the context of data, which obviously is our future," he said. The F.C.C."s chairman, Julius Genachowski, went further, saying the court's opinion "confirms the F.C.C."s authority to promote broadband competition and protect broadband consumers." In 2010, in Comcast v. the F.C.C., the same appeals court rejected the legal theory that the agency was using to validate its regulation of broadband Internet service. While Tuesday's decision does not reverse that ruling, it does signal that the agency may have found a justification for its broadband rules. Both Tuesday's and the 2010 decisions were written by one of the appeals court's more liberal members, Judge David S. Tatel. John Bergmayer, senior staff lawyer at Public Knowledge, which filed a brief supporting the agency, noted that many of the legal arguments Verizon made in the case are also part of another challenge in the same court. There, Verizon is trying to overturn the agency's Open Internet order, a 2011 regulation that contains elements of net neutrality rules. Those rules require Internet service providers to treat all traffic equally, rather than favoring some transmissions over another. The Open Internet case is in its early stages and has not yet been argued before the appeals court. But agency officials say they think the appeals court, in Tuesday's case, rejected at least one of the arguments Verizon makes in the Open Internet case. Many small wireless companies had supported the agency's data roaming requirement, saying that it would provide more competition, particularly by allowing them to offer national service to compete with Verizon and AT&T. The large wireless providers argued that the data roaming order gave them less incentive to invest in their networks, because it would benefit rivals that would not shoulder any of the costs of building infrastructure.