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Watch out: HBO may know you're viewing Game of Thrones illegally

For most fans, April 12 marked the highly anticipated return of "Game of Thrones." However, millions had already illegally downloaded the season's first four episodes by the time HBO aired the fifth season's first episode.

With Game of Thrones one of the world's most pirated television series since it launched in 2011, Time Warner's HBO is now cracking down on those who are watching illegally, according to reports.

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"Game of Thrones"
Source: HBO
"Game of Thrones"

In a forum on social networking site Reddit, Game of Thrones viewers reported receiving emails concerning the downloading of leaked episodes.

One Reddit user posted the full email he had received from Bell Canada, whose online television packages include the HBO Canada channel. The email stated that Bell Canada had received notification that a leaked Game of Thrones episode had been illegally downloaded from the user's IP address.

"The title in question is: Game of Thrones. As the owner of and/or subscriber using the IP address, HBO requests that you immediately take steps to prevent further downloading or uploading of HBO content without authorization," Bell Canada stated in the email.

Bell Canada added that no one was authorized to "exhibit, reproduce, transmit or otherwise distribute HBO Properties" without written consent.

The Reddit user believed that Bell Canada had been contacted by HBO's anti-piracy partner, IP-Echelon. When contacted by CNBC, IP-Echelon declined to comment.

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Jeff Cusson, senior vice president of corporate communications at HBO, told CNBC that the type of emails described by Reddit users were nothing "new" and weren't specific to the four leaked episodes.

"This action is not specific to the four episodes, nor is it new. Notifications to Internet service providers that their users are illegally downloading is standard operating procedure in our digital anti-piracy efforts," Cusson told CNBC on Monday.

Where did the leak come from?

The four episodes leaked from season five "originated from within a group approved by HBO to receive them," according to a statement by HBO this month.

HBO has also been cracking down on Twitter-owned app, Periscope, whose users live streamed the premiere of this season's Game of Thrones. In a statement, HBO said it had issued Periscope with takedown notices.

Those screening episodes in public are under threat too. Brooklyn-based Videology Bar & Cinema, which streamed episodes to customers on Sunday nights for two years, has received a cease-and-desist letter from HBO, according to "Village Voice," a New York news site.

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