Viacom has sued YouTube and its corporate parent Google in a US federal court for alleged copyright infringement and is seeking more than $1 billion in damages.
Viacom claims that the more than 160,000 unauthorised video clips from its cable networks have been available on the popular video-sharing Web site.
The lawsuit marks a sharp escalation of long-simmering tensions between Viacom and YouTube.
Last month, Viacom demanded that YouTube remove more than 100,000 unauthorised clips after several months of talks between the companies broke down.
In a statement, Viacom criticised YouTube's business practices, saying it has "built a lucrative business out of exploiting the devotion of fans to others' creative works in order to enrich itself and its corporate parent Google."
Licencing deal
Viacom said YouTube's business model, "which is based on building traffic and selling advertising off of unlicensed content, is clearly illegal and is in obvious conflict with copyright laws."
Other media companies have also clashed with YouTube over copyrights, but some, including the American broadcasters CBS and NBC/Universal, have reached deals with the video-sharing site to licence their material.
Universal Music Group, a unit of France's Vivendi, had threatened to sue YouTube, saying it was a hub for pirated music videos, but later they also reached a licencing deal with them.
Viacom filed the lawsuit in the US district court for the southern district of New York and is also seeking an injunction prohibiting Google and YouTube from using its clips.