Fear of legal action from the Recording Industry Association of America (RIAA) may be behind a fall in the number of people downloading music files from peer-to-peer (P2P) sites.

According to market research from The NPD Group, the number of people downloading music dropped from 14.5 million in April to 10.4 million in June.

NPD said it couldn't prove that the fall was due to the RIAA's decision to sue individuals for copyright infringement, but pointed out that the drop appeared to coincide with the trade body's legal campaign.

But the survey of 40,000 users also showed that the average number of files downloaded per user grew between April and June from 59 to 63.

The RIAA's campaign did not start until late June, but NPD suggested that the fall-off in numbers seemed more than the usual seasonal downturn.

Summer and other holiday seasons normally see a dip in the number of people using P2P sites, as many are away and students are deprived of the fast connections at universities.

Russ Crupnick, vice president at NPD, said in a statement: "While we can't say categorically that the RIAA's legal efforts are the sole cause for the reduction in file acquisition, it appears to be more than just a natural seasonal decline.

"This decrease is sharper than the declines we're seeing in the offline retail world.

"In addition, because the initial drop followed well-publicised legal efforts, there is evidence to show that the RIAA's tactics may be having the desired impact on file sharing among consumers."

The RIAA said recently that it only plans to target people that have distributed "substantial" numbers of copyrighted files.

"In fact, we know that it's really only about five or six per cent of all the file traders who download and swap about half of all the files online," said Crupnick.

"But as an individual, you don't know whether 20 downloaded files or 2,500 is a lot because you don't have that baseline."