The Federal Communications Commission has ordered Comcast to stop blocking peer-to-peer traffic by year's end, change its network management practices, and comply with several other requirements or face the possibility of an immediate injunction.

The FCC published a letter, statements from commissioners, and a press release on its Web site Wednesday, saying that Comcast's network management practices contravene federal policies that protect "the vibrant and open nature of the Internet." It also set forth a list of requirements the company must meet within 30 days.

Sena Fitzmaurice, senior director of corporate communications and government affairs for Comcast, declined to comment on the details of the order.

"We're reviewing the order and evaluating our options," she said during a brief interview Thursday.

Comcast customers had complained of problems using peer-to-peer applications like BitTorrent. The public interest groups Free Press and Public Knowledge lodged formal complaints with the FCC and petitioners requested a declaratory ruling against the company.

Comcast denied blocking the applications when complaints first surfaced, but The Associated Press and the Electronic Frontier Foundation ran tests and announced that Comcast did appear to interfere with the peer-to-peer software.

"Comcast changed its story and admitted that it did target its subscribers' peer-to-peer traffic for interference," the FCC explained in its statement. "The company initially claimed that it did so only during periods of peak network congestion and of heavy network traffic. Later, confronted with yet more evidence suggesting that interference was not limited in this manner, Comcast recast its position yet again and admitted that it interferes with peer-to-peer traffic regardless of the level of overall network congestion at the time and regardless of the time of day."

The FCC investigated and held hearings on the issue. This week, it announced that "Comcast has unduly interfered with Internet users' right to access content and applications" of their choice.

"Comcast's interference is far more invasive and widespread than the company first conceded," the FCC said.

The FCC said Comcast deployed network equipment to monitor its customers' content and block peer-to-peer connections.

"The Commission found that Comcast monitors its customers' connections using deep packet inspection and then determines how it will route some connections based not on their destinations but on their contents," the FCC wrote. "In essence, Comcast opens its customers' mail because it wants to deliver mail not based on the address on the envelope but on the type of letter contained therein. The Commission also found that Comcast's conduct affected Internet users on a widespread basis. Indeed, Comcast may have interfered with up to three-quarters of all peer-to-peer connections in certain communities."