Most popular search terms on BearShare

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It also warned that children could come across pornography by accident.

When congressional investigators tried to use one of these services to download videos of teen popstar Britney Spears, 70% of the files they found were pornographic and had nothing to do with the singer.

"This is an alarming evolution of the porn industry," said Mr Largent.

File-sharing software flourishing

The congressional investigation began about two months ago, after a parent called Democrat Henry Waxman, saying that his son was able to freely download pornographic videos from the internet, using a file-sharing service called BearShare.

The latest version of BearShare includes a family filter, which the company says will hide most search results with adult content.

Programs like Bearshare have flourished since Napster was effectively shut down.

These services, many of them on the Gnutella file-sharing network, allow users to search and download files from each other's computers.

While Napster only let you swap MP3 music files, programs like BearShare can be used to trade images and video.

Searching for 'XXX'

The investigators said a search for the word "porn" on BearShare results in more than 25,000 entries, many of them video files.

"Porn", "sex" and "xxx" were among the 10 most popular search terms used, said the report.

File-sharing programs are growing in popularity, as users look for alternative ways of trading music and other material over the internet.

A leading internet clearinghouse recently revealed that more than 3.1 million copies of file-sharing programs were downloaded from its site in just one week, according to the report.