US cable provider Comcast is interfering with BitTorrent file transfer traffic for its broadband subscribers.

A test by Associated Press demonstrated that the provider prevented users from sharing files on the file sharing network. The firm achieves this by sending a message to the computer requesting the file which instructs it to stop communicating.

Even though it is common knowledge that BitTorrent is used for downloading copyrighted materials on a massive scale, the network also has plenty of legitimate applications. Several Linux distributions including Ubuntu, for instance, rely on the network to dispense their software. In the Associated Press test, the agency attempted to share and download a copy of the King James Bible, which is in the public domain.

Comcast is the second largest internet provider in the US. The firm previously denied that it blocked or discriminated against BitTorrent traffic.

Comcast claims that the upload-blocking measure is intended to ensure network uptime for all users. The video and audio files that are up- and downloaded through the file sharing network take up the vast majority of all network traffic, and could degrade network speeds for other users. Comcast's defence, however, sparked some questions, given that the tested file measured only a few megabytes.

Instead, it rekindled the debate about net neutrality, which revolves around the question of whether a provider should be allowed to arbitrarily block content. Although vendors hold the legal reign over their own networks, critics caution that their commercial interests could interfere with free speech or open competition.