By Andy Sullivan

WASHINGTON (Reuters) - Hollywood took its fight against Internet song-swapping to the office on Thursday, asking the 1,000 largest U.S. public companies to block employee use of online file-trading networks.

Trade groups representing record labels, movie studios, songwriters and music publishers said they plan to send a letter this week to corporate chieftains asking them to prevent employees from using "peer to peer" networks such as Kazaa and Gnutella that enable users to download songs, movies and other digital files for free.

"We urge you to take whatever steps necessary to ensure that your network is not being misused to infringe copyrighted works," the letter said. "Using technology to steal music and movies is no different from walking into a store and shoplifting a CD or DVD."

The letter comes on the heels of a similar request sent to college and university administrators several weeks ago. Like college campuses, many offices are wired with high-speed Internet access, allowing large files such as digital video to be downloaded quickly.

By contrast, roughly nine out of 10 home Internet users rely on slower, dial-up service.

The letter suggest that businesses could be held liable for copyright infringement on their corporate networks and asked that corporate legal officers be made aware of their request.

It was signed by the Recording Industry Association of America, the Motion Picture Association of America, the National Music Publishers' Association and the Songwriters Guild of America.

The music industry blames Internet-based song-swapping for a decline in sales and has moved aggressively to shut down peer-to-peer networks since Napster burst on the scene in 1999.

Industry lawsuits knocked Napster offline last year, but users turned to a wide array of other services that sprung up in its place.

The music industry has since switched tactics and sought to convince users that their actions are immoral and illegal through an advertising campaign and a speech during the annual Grammy Awards ceremony.

The industry has hired software companies to track down individual users, drawing the ire of Internet providers that do not want to turn over the names of subscribers who use such networks.

Record companies have also tried to gum up peer-to-peer networks with dummy files and traffic-slowing measures and have enlisted the support of the Justice Department.