The Recording Industry Association of America (RIAA) has filed its largest number of lawsuits to date, slapping notices on 532 alleged users of peer-to-peer (P2P) sites.

The total issued since September 2003 has now nudged over 1,000, many of which have been settled out of court.

People who come to an agreement with the RIAA, and promise to cease uploading files to P2P sites, have agreed settlements of around $3,000 each.

But the RIAA is warning that, although it will come to agreements with people who want to settle out of court, the amount they will have to pay could become substantially larger.

This is because the US Court of Appeals ruled in December that the RIAA could not go directly to ISPs with subpoenas to force them to reveal file sharers' names.

Now the RIAA has to file so-called 'John Doe' lawsuits, a more time consuming and expensive process, so it is likely that the RIAA will pass these costs onto the P2P users.

Because the identity of the P2P user is not known, other than by their IP address, the RIAA files a lawsuit against 'John Doe' until the identity can be determined.

After the suit is filed, the judge can grant the RIAA's request for a subpoena to order the ISP to hand over the individual's identity and details.

This also means that the RIAA will not be able to notify people before the lawsuits are filed as they have done in the past.

The trade body claimed that it was taking legal action to give the legitimate music sites, such as the new Napster, a chance to succeed.

"We will continue to file lawsuits against major file sharers of copyrighted music," stated RIAA president Cary Sherman.

"The process by which we identify defendants has changed. The programme has not."