Comcast, the nation's largest cable company, violated federal guidelines when it blocked and degraded Web traffic, the head of the Federal Communications Commission will announce Friday.

The sanctions would be the first time the commission has come down on an internet provider for denying consumers the right to open, unfettered internet access. It may set a precedent on how the federal government oversees management of internet traffic flows in the future.

Last fall, Comcast reluctantly acknowledged that it had temporarily blocked certain peer-to-peer traffic (file sharing). The cable giant called its actions "reasonable network management."

But consumer rights groups and internet experts accused the company of violating the F.C.C.'s 2005 "Internet Policy Statement," which established four principles intended to guarantee consumers unfettered access to all legal Web content, applications, and services.

The chairman of the F.C.C., Kevin Martin, now agrees.

"The commission has adopted a set of principles that protects consumers' access to the internet," he told the Associated Press on Thursday night. "We found that Comcast's actions in this instance violated our principles."

The consumer advocacy group Free Press trumpeted Martin's decision as a victory for consumers.

"This is going to be a bellwether," said Ben Scott, federal policy chief for Free Press.

The decision, contained in an order to be circulated by Martin, brings the agency's nine-month investigation of Comcast close to completion. Martin, a Republican, is expected to gain support from the two Democratic F.C.C. commissioners for his position, which would ensure the order's passage when the commission meets on August 1.

Comcast has long maintained that the government's standard gives it the right to manage its digital traffic "reasonably" for the sake of "network management."

For almost a year, consumer rights groups have battled Comcast, after an Associated Press investigation discovered that Comcast was blocking legal peer-to-peer traffic.

Comcast faced further public outrage after it admitted to paying people off the street to sit at a public hearing at Harvard, while members of the public were prevented from attending. At the time, Comcast claimed it merely paid people to save spots at the hearing for Comcast employees, but the event's organizer disputed that claim.