Comcast, the nation's largest cable company, sued the Federal Communications Commission on Thursday, challenging the regulator's authority to sanction it for violating government internet rules.

Comcast's lawsuit sets up a legal showdown over whether the F.C.C. has the power to enforce "network neutrality"—the principle that all Web traffic should be treated equally—or whether new legislation is needed by Congress to give it such authority.

The F.C.C. voted 3-2 last month to punish Comcast after concluding that the cable giant had run afoul of the agency's 2005 Internet Policy Statement, which made network neutrality official government policy.

Comcast has long argued that the F.C.C. lacks the authority to enforce that principle and, as widely expected, today formally took its case to the U.S. Court of Appeals in Washington, D.C.

"We filed this appeal in order to protect our legal rights and to challenge the basis on which the commission found that Comcast violated federal policy in the absence of pre-existing legally enforceable standards or rules," Comcast executive vice president David Cohen said in a statement.

• Comcast's petition can be found here.
• A timeline of the Comcast probe is here.

Even as it filed the lawsuit, Comcast said it intends to comply with the F.C.C. orders, which "essentially codify the voluntary commitments that we have already announced."

The company does not dispute that the F.C.C. has the authority to regulate internet service providers, but said "we are compelled to appeal because we strongly believe that, in this particular case, the commission's action was legally inappropriate and its findings were not justified by the record."

Unchallenged, the F.C.C. order would have set a precedent giving it the right to enforce its 2005 policy. Comcast says its suit is less about the particulars of its case, and more about the scope of F.C.C. authority moving forward.

Throughout the government probe, Comcast maintained that its actions amounted to "reasonable network management" designed to ensure that its network functions smoothly.

The company said it was forced to occasionally block peer-to-peer traffic, which it said took up a vastly disproportionate amount of its network and caused congestion, to the detriment of other users.

In March, facing intense F.C.C. and public pressure, Comcast said it would move to a "protocol agnostic" network-management approach. That would focus not on a particular type of network activity—such as peer-to-peer traffic—but rather on those users who consume the most bandwidth.

Network neutrality advocates, who have long battled Comcast over its network management practices, called Comcast's lawsuit against the F.C.C. predictable.

"We expected Comcast would appeal the commission's order," said Gigi Sohn, president and co-founder of Public Knowledge, a digital-advocacy group. "The company opposed it every step of the way, even as they failed to disclose their throttling of internet traffic. We believe the commission will prevail and the rights of internet users will be protected."

Ben Scott, policy director of Free Press, which filed the original complaint against Comcast last fall, said he wasn't surprised by Comcast's lawsuit, and expressed support for the F.C.C.

"Comcast's appeal is predictable—the cable giant has a long history of appealing any decision it doesn't like. The F.C.C. is well within its authority to protect the open internet, either by adopting rules or acting on complaints," Scott said.