Game of Thrones ... Emilia Clarke as Daenerys.

Game of Thrones proves popular with pirates. Above: Emilia Clarke as Daenerys.

The critically acclaimed US fantasy drama Game of Thrones has topped the chart for illegal downloads, confirming it is the most popular TV show watched by people who are not watching TV.

More than 4.2 million people around the world downloaded the series - the chart is calculated by ranking the most downloaded single episodes of shows - roughly matching its US television audience.

The series draws a weekly audience of around 3.8 million. Its second season finale, which aired in the US in June, attracted around 4.2 million viewers.

Once Upon A Time.

Legally popular ... Once Upon A Time.

The list, compiled by the website TorrentFreak from a number of sources, including reports from public BitTorrent trackers, represents an eclectic mix of shows.

It includes popular shows fuelled by critical mass (Big Bang Theory, How I Met Your Mother), blue-chip cable dramas not always available outside the US (Breaking Bad, The Walking Dead) and cult shows which appeal to the geek demographic (Fringe, Revolution).

Illegal downloading via BitTorrent networks has been in slight decline for some time, though reports suggest 2012 has seen a small rise. In part that would be fuelled by faster internet services being introduced worldwide and a migration away from traditional television in which some consumers now exclusively watch TV content via the internet.

Richard E. Grant in the Doctor Who Christmas special.

Much anticipated ... Richard E. Grant in the Doctor Who Christmas special.

In the US, where the phenomenon has attracted some discussion, they are referred to as "cord cutters" or "cable cutters", because they are households which have effectively cut their cable cord. (Many homes in the US get terrestrial television via cable, as well as pay TV.)

The majority of TV-show piracy - a figure estimated to be around 80 per cent - occurs outside the US.

Australia has been identified as a major source of illegal downloading.

But Game of Thrones's numbers would be bolstered by larger-than-usual illegal downloading inside the US. The series is not offered for free by HBO as an online playback, and HBO's digital product, HBO GO, is only offered bundled with the channel subscription.

As a result, many US fans of the series have turned to the internet and illegal downloading to see the series.

Piracy has a much smaller impact on more mainstream US shows, such as Modern Family, NCIS and CSI, because most US networks offer their content for free to US viewers via their websites after they are broadcast on TV.

In contrast, there is no collective measurement for legal downloading as most broadcasters, in Australia and the US, put legal content online via their websites but do not publish comprehensive viewing data.

A list published by the US iTunes store in June 2012 of the most popular legally downloaded TV programs included Dexter, Breaking Bad and The Walking Dead. But other more mainstream shows, such as Doctor Who, Modern Family, True Blood, Top Gear and Glee were also prominently positioned.

The Australian iTunes store currently ranks Gossip Girl, Homeland, The Vampire Diaries, Grey's Anatomy, Once Upon A Time, Hart of Dixie, Modern Family and Hunted among its most popular single TV episodes.

The top-ranked season pass in the Australian iTunes store is for Doctor Who's upcoming Christmas special, The Snowmen, which is not available until Boxing Day. (It airs in the UK on Christmas Day.)

Its position at the top of the iTunes leaderboard would suggest pre-sales of the special - it is priced in Australia at A$4.99 - have been strong.

Doctor Who is also one of the most popular offerings on the ABC's iView platform, an online and app-based TV service. It is the market leader for online TV playback in Australia largely because of its easy-to-use interface and a lack of commercial competition.

Foxtel also offers internet-based services via the Xbox 360, T-Box and its Foxtel Go iPad app, a multi-channel tablet interface which was based on its almost universally acclaimed London 2012 Olympics iPad app.

Full list: Most popular illegally downloaded TV shows

1     Game of Thrones           4,280,000 (downloads)   
2     Dexter                         3,850,000   
3     Big Bang Theory            3,200,000    
4     How I Met Your Mother   2,960,000   
5     Breaking Bad                 2,580,000  
6     The Walking Dead          2,550,000  
7     Homeland                     2,400,000  
8     House                          2,340,000   
9     Fringe                          2,280,000    
10   Revolution                     2,130,000