Illegal music downloading by American consumers fell last year, continuing a trend from 2005, a survey showed.

The survey from NPD Group found that the number of consumers using peer-to-peer services to download music declined 17 percent in 2012.

When file sharing peaked in 2005, one in five Internet users aged 13 and older, or 33 million people, used peer-to-peer services to download music, but fell to 21 million last year.

The volume of illegally downloaded music files declined 26 percent, the survey found.

According to NPD, 40 percent of US consumers who had illegally downloaded music in 2011 reported that they had stopped or downloaded less music from these networks.

The biggest reason for this reduced sharing activity was an increased use of free, legal music streaming services, NPD said.

"For the music industry, which has been battling digital piracy for over a decade, last year was a year of progress," said Russ Crupnick, senior vice president at NPD.

"Among other factors, the increased use of legal and licensed streaming services has proven to be an alternative for music fans who formerly used P2P networks to obtain music."

NPD said another factor in the decline in illegal music downloading was the demise of LimeWire, shut down by a US federal court in 2010 following a lawsuit filed by the music industry.

The survey was released as US Internet providers prepare to implement a new "copyright alert" system to curb piracy of music, film and other content.

The program, created with the music and film industry and the largest Internet firms, with some prodding by the US government, will include notices to consumers suspected of illegal downloads.

The Center for Copyright Information, which is coordinating the effort, said the Internet firms are starting to roll out the system this week.

Each Internet firm is devising its own policy, but in some cases consumers may see their Web access throttled or be required to visit information pages on copyright violations.