It has gone from the realm of geeky fandom to a worldwide cult TV hit in the time it takes to swing a mighty broadsword. Yet when the latest instalment of Game of Thrones, the acclaimed sword and sorcery adventure series, hits British screens tomorrow night there is a good chance that the keenest fans will have already seen the first episode.

Based on George R R Martin's best-selling series of novels, the show has become a victim of its own success and is now one of the most illegally downloaded TV series of all time, with one episode having been shared more than four million times – roughly equal to the number of legal viewers it now attracts.

Taking place in the fictional, war-ravaged realm of Westeros, the epic tale – sometimes jokingly referred to as The Sopranos in Middle Earth – is loosely based on the Wars of the Roses and features X-rated levels of sex and violence.

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But critics have fallen over themselves to sing the praises of the series, with its Hollywood-style big budget and an intelligent and complex script. The predominately European cast, including Sean Bean and Charles Dance, is one of the largest ever gathered for a TV series.

It has even made stars out of the unlikeliest of people: Wilko Johnson, formerly a member of the 1970s R&B band Dr Feelgood who was diagnosed with terminal cancer earlier this year, plays a mute knight and executioner.

Filmed predominately in Northern Ireland and Malta, the lavishly produced drama – the producer HBO spent $60m (£39m) on the first season alone – pivots around the competing royal ambitions of two powerful dynasties, the Lannisters and the Starks.

The anti-piracy group Federation Against Copyright Theft (Fact), the UK's leading trade organisation protecting intellectual property rights, described this trend as not only damaging to companies investing in the series, but detrimental to local economies benefiting from the show. "HBO has helped create hundreds of jobs in Northern Ireland where parts of Game of Thrones were filmed, and has a direct positive impact on the people who live there."

However, the show's director, David Petrarca, downplays the effects of piracy, saying Game of Thrones has actually "thrived from the cultural buzz" generated by more people having access to the programme.

Laurence Kaye, leader of the UK Pirate Party, which describes itself as "a democratic political party standing for digital rights and civil liberties", agrees, and claims that a fairer deal for UK viewers would reduce the number of people pirating the series.

"It is heavy-handed intellectual property regulation and licensing that is holding companies back," Mr Kaye said. "It is ludicrous that fans actually have to form a group called 'Take My Money, HBO!' because the only way they can see Game of Thrones is by pirating."

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Currently, Game of Thrones fans are forced to buy packages starting at £36 per month with Sky to watch the series.

But Mr Kaye believes a fairer deal for viewers would increase interest in legally purchasing content from the show's owners.

"There are significant gaps in on-demand services in the UK, both in terms of content and different delivery methods. There is no licensed channel to watch it on demand without buying an expensive package from a broadcaster you might not want to support," he added.

A growing tribe of fans

The US hit series Game of Thrones returns for its third series tomorrow, much to the delight thousands of British fans, who can count Stephen Fry and Jonathan Ross among their ranks.

Its appeal is so great that some devotees take their passion further by dressing up as their favourite characters, organising conventions and even tattooing Game of Thrones icons on their bodies.

"The show instantly drew me in as I enjoy medieval-based shows and movies," said Robert Graver, 20, from Southampton.

"I decided to offer my body to my local tattoo artist for him to test some designs on. I plan on getting another tattoo of Arya Stark surrounded by a Direwolf, and am keen on adding three dragons around my current Daenerys Targaryen piece."

Phil Lowles, organiser of the forthcoming convention TitanCon, which will be held in Belfast this September, describes how fandom goes beyond love for the series itself. "You've got to have that sense of community for things like TitanCon to happen, and the Game of Thrones fandom has that in spades," he told The IoS.

"There is a feeling you get when you meet someone else in the same fandom – a feeling of belonging, like you've come home or found your tribe. I'm sure fans of football teams or musicians get those same feelings of finding people with a similar mindset, people they can make lifelong friendships with."

Tom Alvarez