Game of Thrones fans who did not want to wait and legally watch HBO's hit series premiere its fifth season tonight have leaked the first four episodes online. Online pirates have unveiled nearly  half of the show's next season, which is scheduled to debut its first episode tonight. The episodes began appearing on torrent download sites on Saturday night, and are thought to have leaked from review copies given to the press. Fans of the show, which follows the ambitions and betrayals of competing noble factions and families in the fantasy world of Westeros, now face the daunting task of avoiding spoilers online as the first four episodes are released over the course of the month. Scroll down for video . Game of Thrones, which includes characters such as Emilia Clarke's Khaleesi (left) and Nathalie Emmanuel's Missandei (right), was pirated before its season five debut tonight . Episodes beginning this year's season are titled The Wars to Come, The House of Black and White, High Sparrow and The Sons of the Harpy. They had been downloaded more than 550,000 times as of early Sunday morning, Variety reported, citing the piracy-tracking company Excipio. 'Sadly, it seems the leaked four episodes of the upcoming season of Game of Thrones originated from within a group approved by HBO to receive them. We're actively assessing how this breach occurred,' HBO said in a statement to Daily Mail Online. The series is the most illegally downloaded program in the world. Anti-piracy company Irdeto said that episodes from Game of Thrones's first four seasons were illegally downloaded more than seven million times between February and April 2015. The astounding figure is a 45 per cent jump over the same period from 2014 for the show, which debuted in 2011. The first four episodes had been downloaded more than 550,000 times as of early Sunday morning. Above, Kit Harrington's character Jon Snow (left) and Peter Dinklage's Tyrion Lannister (right) feature in the show . Brazil and France are the top two countries by illegal downloads of the series inspired by George R R Martin's Song of Ice and Fire novels. They are followed by the US, Canada and the UK. Walking Dead, with 5.7million illegal downloads from February to April, was the second-most pirated show. HBO planned Sunday's premier to air across 170 countries at the same time in an effort to combat international piracy of the show. US audiences are able to watch Game of Thrones on HBO Now, the company's new online streaming service.
Episodes for the next four weeks released Saturday night on pirate sites.
Illegal material thought to have leaked through press review copies.
Show is the most pirated in the world, with Brazil the worst offender.