Comcast is testing new technologies that would slow the transmission of Internet files for its biggest users by as much as 20 minutes during times of heavy network congestion. But the nation's largest cable provider has promised not to target specific content, such as video files that compete with its cable television business.

The tests come as the Federal Communications Commission yesterday released an order that forces Comcast to stop its earlier efforts to block transmission of certain Internet files, a ruling that public interest groups hailed, saying it would prevent network operators from acting as gatekeepers of the Web.

Comcast didn't respond to details of the FCC's order, but spokeswoman Sena Fitzmaurice said, "We are examining the order and will evaluate our next step."

Comcast began testing its system of slowing certain traffic in March and has expanded the tests to Warrenton; Chambersburg, Pa.; Colorado Springs; Lakeview, Fla.; and East Orange, Fla. It will adopt the new network management technology by the end of the year, the company said.

"We are in a trial, and final results of that are yet unknown, which is why we do trials," said Mitch Bowling, Comcast's general manager of online services. "The important point is that the intent of network management has and continues to be to provide a great experience for our customers."

In yesterday's order, the FCC concluded that Comcast's earlier management of Internet traffic was discriminatory and "inconsistent with the concept of an open and accessible Internet."

After public hearings in Cambridge, Mass., and Silicon Valley and several months of investigations, the agency said Comcast had looked into the packets of Internet files being transferred between users and deliberately blocked certain files.

The commission said Comcast had an "anticompetitive motive" because it delayed and blocked peer-to-peer files through applications such as BitTorrent. Such files often are high-quality video that might otherwise be watched and paid for on cable television.

The agency ordered Comcast to stop the practice and gave it 30 days to provide details on how it manages Web traffic and come up with a plan describing how it will stop the practice.

FCC Chairman Kevin J. Martin wrote in a statement that the order was "another important step to ensure that all consumers have unfettered access to the Internet." He compared Comcast's blocking of Internet content to the post office secretly opening mail and deciding not to send it.

Martin, a Republican, voted with the two Democratic commissioners, Michael J. Copps and Jonathan S. Adelstein, to adopt the order against Comcast. The other two Republican commissioners, Robert M. McDowell and Deborah Taylor Tate, voted against the order.

"The framework we adopt today will send a message to the industry that bad actors will be punished," Martin said in the statement.

Comcast has maintained that management of traffic among its 14.4 million Internet users is nondiscriminatory. Because the network is shared at the neighborhood level, the company said, it must manage the flow of Internet traffic to ensure that Web access is not slowed.

The public interest groups Free Press and Public Knowledge filed a complaint with the FCC after users discovered that Comcast was delaying the traffic of direct file shares by BitTorrent.

"With this order, the Internet remains in the hands of [consumers] and the greatest engine of free speech and commerce since the printing press," said Ben Scott, Free Press's policy director.