A common loophole on ringtone websites means many people are downloading popular tunes without paying a penny, research shows.
Web users get the tones without paying a few clicks
Ringtone sellers could be losing more than 50m euros (£34.7m) a year through the loophole, said security firm QPass.
Almost one-third of 100 European websites selling ringtones were vulnerable to the security lapse.
QPass predicts that losses will grow as people look to download ever more types of data to their hi-tech handsets.
Ringtones are hugely popular with mobile owners.
In some European nations they generate more cash than sales of music singles. It is rare now that a top selling pop song does not have an accompanying ringtone.
QPass surveyed the websites of 42 mobile operators and 58 online stores in Europe selling ringtones to find out which were doing a good job of protecting the downloadable tones.
Steve Shivers, a senior vice-president at QPass, said many websites allow tunes to be previewed before being bought and in doing so inadvertently gave people a chance to copy them to a PC.
Once downloaded, the ringtones can be easily transferred to a handset through a direct cable connection, on a memory card or via Bluetooth short-range radio technology.
In total 35% of the sites tested by QPass let people get ringtones through this preview mechanism.
The websites tested in some countries were more secure than others.
Only 20% of UK websites were at risk but 60% of German sites selling ringtones were vulnerable. Paradoxically the ringtone market in Germany is the biggest in Europe.
"It requires nothing wacky or tricky," said Mr Shivers. "It's not like you are exploiting some sort of trap door that's a super-secret way of doing this."
"They always pick the best riff or the chorus from a song as a preview so it's always packaged as you would want to it," he added.
Andrew Bud, vice chairman of the Mobile Entertainment Forum which represents many ringtone sellers, said it was more likely to be a problem for smaller firms.
"The big players do know what they are doing and don't suffer from this problem," he said.
The reason so many websites are leaving themselves open to this abuse was because sales of ringtone downloads have grown much faster than anyone expected.
"A sloppy experiment in mobile commerce has turned into a huge business and the operators were not ready," said Mr Shivers.
"They are using a patchwork of systems kludged together as the market has grown."
Over the next three years QPass estimates that these free downloads could cost website operators 336m euros (£234m) in lost revenue.