Comcast is poised for legal showdown with the Federal Communications Commission as the agency moves to fine the cable giant for blocking file-sharing traffic on its Web network.

Two commissioners, Democrats Michael Copps and Jonathan Adelstein, late last week joined Chairman Kevin Martin to vote in favor of sanctioning Comcast, giving the commission a majority in favor of punishing the cable giant. A formal vote is set for Friday.

The F.C.C.'s ruling could represent a landmark victory for "network neutrality" advocates and set a precedent for the rules governing how internet service providers are allowed to manage their Web traffic.

Network neutrality, the principle that all Web traffic should be treated equally, was enshrined in the F.C.C.'s 2005 Internet Policy Statement, which established four principles intended to guarantee consumers open access to all legal internet content.

Comcast argues, however, that the commission lacks the authority to enforce those principles.

"In order to enforce something, an agency must be enforcing a rule that has force of law," Comcast spokesperson Sena Fitzmaurice told Portfolio.com. The F.C.C.'s internet policy statement does not meet that test, the company believes.

F.C.C. Chairman Kevin Martin, however, believes the enforcement of the internet policy is within the agency's purview.

"I am pleased that a majority has agreed that the Commission both has the authority to, and in fact will, stop broadband service providers when they block or interfere with subscribers' access," Martin said in a statement Saturday.

The battle over whether the F.C.C. can fine Comcast for violations of the internet policy could land in federal court if the company decides to challenge the ruling.

Last year, a consortium of consumer-rights groups led by Free Press filed a complaint after Comcast was found to be interfering with peer-to-peer Web traffic. Comcast initially denied the charge before admitting that it had been practicing what the company called "reasonable network management," as allowed by the F.C.C.'s policy statement, in an effort to prevent Web traffic bottlenecks.

But three out five F.C.C. commissioners now disagree with the company's position, giving the panel a majority vote to punish the company.

In a statement, Comcast continued to defend what is called its "reasonable network management" practices.

"We continue to assert that our network management practices were reasonable, wholly consistent with industry practices, and that we did not block access to websites or online applications, including peer-to-peer services," Comcast said. "We do not believe the record supports any other conclusion."

Network neutrality advocates hailed the news of the F.C.C.'s ruling.

"This vote reflects the bipartisan support for protecting consumers' access to the free and open internet," said Marvin Ammori, general counsel of Free Press and author of the original complaint. "Comcast's blocking is a flagrant violation of the online rights established by the F.C.C. If adopted, this order would send a strong signal to the marketplace that arbitrarily interfering with users' online choices is not acceptable."