The music industry is stepping up its campaign against piracy
The record industry has issued a severe online warning to internet users over music piracy.
Song swappers have been greeted with an online message telling them they could be "easily" identified and face "legal penalties".
About 200,000 users of the Grokster and Kazaa file-sharing services received the notice as the music business stepped up its campaign against "music pirates".
The Recording Industry Association of America (RIAA) - the trade group for the world's biggest music companies - said millions more users would get notices in coming weeks.
When you offer music on these systems, you are not anonymous and you can easily be identified
Recording Industry Association of America
Its message read: "It appears that you are offering copyrighted music to others from your computer. When you break the law, you risk legal penalties.
"There is a simple way to avoid that risk: don't steal music either by offering it to others to copy or downloading it on a file-sharing system like this."
It added: "When you offer music on these systems, you are not anonymous and you can easily be identified."
The RIAA represents the music industry's biggest players - including Universal, Warners, EMI, BMG and Sony.
It said it was using song titles to identify users who were posting copyrighted songs for others to download.
Sharman Networks Ltd, which owns Kazaa, attacked the move.
It said that rather than co-operating with the file-sharing network, "the RIAA continues to choose to attack some of its most loyal customers".
The warning has also angered some Internet users.
One posted a message on Yahoo saying:
"Way to go, RIAA. Sue and threaten the public, your customers. I think I'll go and download."