Actor Peter Dinklage is shown in a scene from the HBO series "Game of Thrones" in this undated publicity photograph. (REUTERS/Courtesy HBO/Handout)

Game of Thrones topped a list of most pirated TV shows in 2012.

HBO's hit fantasy series based on George R.R. Martin's bestselling books saw 4.28 million downloads for a single episode, said popular pirating site TorrentFreak.

And illegal TV pirating is on the rise, says TorrentFreak, citing airing delays and a lack of accessibility. For example, 80% of TV pirating comes from outside the U.S., and a big chunk of Game of Thrones downloaders are from Australia, where the show airs a week later than in North America.

"In common with many branches of the entertainment industry, the challenge for TV companies such as HBO is to come up with a business model that allows users to consume what they want at any given time, without losing existing revenue in the process," said TorrentFreak. "When that puzzle is solved worldwide the number of unauthorized downloads will be pretty much insignificant."

Game of Thrones is followed by Dexter, last year's No. 1 downloaded show, at 3.8 million.

"Episodes of both shows are shared among millions of people, with airing delays and limited legal download options among the key reasons to pirate," said TorrentFreak.

Third place goes to The Big Bang Theory at 3.2 million, How I Met Your Mother at 2.96 million, Breaking Bad at 2.58 million, The Walking Dead at 255 million, Homeland at 2.4 million, House at 2.32 million, Fringe at 2.28 million and Revolution at 2.13 million.