The Federal Communications Commission, citing concerns about national security, has abandoned a 10-year-old policy and will no longer give the public access to information about past telephone network outages.

The decision, reached this month, has angered consumer advocates and some state regulators who say the data are a critical tool in evaluating phone service reliability around the country. They also say that concerns that the information poses a national security risk are overstated.

The reports typically include details about the cause of an outage, how long it lasted and how many customers were affected. The information is used by state regulators, consumer groups and consultants to assess which companies are having problems and which are most reliable. Large companies use the information to make decisions about where they build their own networks and to plan for key facilities such as data centers.

The information "is essential to your ability to understand what is going on," said Brian R. Moir, a lawyer who represents large business customers.

According to Moir, most of the information is mundane, providing details about the fallen tree limbs that knock out telephone poles or construction crews that accidentally sever fiber optic lines. "If you look at 99 percent of incidents that trigger reports, they are not what I call, Osama [bin Laden] issues," Moir said.

But an FCC official who was involved in the decision and who spoke only on the condition of anonymity said a report of a downed line that results in a major outage could highlight for terrorists points of weakness in a large network.

Although the FCC will not make the information it collects about the outages public, the companies are free to keep the public informed about outages as they are occurring.

While the FCC limited the amount of information it is making public, it has expanded the amount of information it collects from the wireless and satellite industries. Previously, cell phone and satellite companies reported information about outages on a voluntary basis.

Under the new rules, wired, mobile and satellite carriers must report an outage that affects 900,000 user minutes or more. For example, an outage that affects 90,000 customers would have to be reported if it lasted 10 minutes or more.

FCC officials said the new rules are a compromise allowing the agency to expand the information it collects without endangering national security. "What the national security guys are telling us is that it is a mistake going forward to make outage report[s] available publicly," said Fred Thomas, chief of staff with the FCC's Office of Engineering and Technology.

California State Public Utility Commissioner Carl W. Wood said there is lots of information that the companies could make public but don't for competitive reasons. For instance, many cell phone companies decline to release data on their signal strength in different areas. If the information were public, consumers could pick their mobile carrier based on the strength of their local signal, Wood said. "They absolutely do not want to compete on the basis of quality."

Moir said he has been lobbying the FCC for many years to expand its outage reporting requirements beyond traditional wired telephone companies. He won the battle only to have the agency make the information confidential. But Moir said that he will continue to press the FCC to change its mind about keeping the data out of the public's hands.

Privacy experts said the FCC's decision to make previously public information secret follows other steps taken in the wake of the terror attacks of Sept. 11, 2001. "It's certainly been a trend since September 11 to make less information available under circumstances that are questionable," said David Sobel, general counsel for the Electronic Privacy Information Center, a Washington public interest group.