Comcast's actions, first reported by the Associated Press last October, appeared to "block uploads of a significant portion of subscribers" in that part of the network, even during times when the network wasn't congested, Martin told a Senate committee. "Based on testimony we've received thus far, this equipment was typically deployed over a wider geographic area or system, and is not even capable of knowing when an individual ... segment of the network is congested."

The study found BitTorrent interference from 11 other Internet services providers, in addition to Comcast and Cox, with seven of those in the U.S. But there was not "widespread" BitTorrent blocking at those ISPs, the study said. The tests looked at 1,224 ISPs worldwide.

Advocacy groups the Open Internet Coalition, Public Knowledge and Free Press all pointed to the study as evidence that the U.S. Congress needs to pass net neutrality legislation prohibiting broadband providers from blocking or slowing Web content.

"Consumers have no reason left to trust their cable company," Ben Scott, policy director of Free Press, said in an e-mail. "[The] sophisticated testing shows that Comcast and Cox block BitTorrent applications at all times of the day -- not just at times of peak traffic. Now is the time to send a clear signal to the market that blocking consumers' access to the lawful Internet content of their choice is out of bounds."