HBO, the company behind TV blockbuster Game of Thrones is assessing how several episodes were leaked on the internet a full 24 hours before the first episode of season five was due to broadcast.
The first four episodes of the new season, much of which was shot at locations in Northern Ireland, appeared on torrent sites and were downloaded hundreds of thousands of times via sites including The Pirate Bay.
Variety magazine reported that downloads exceeded 1.7 million worldwide by 5pm on Sunday.
TorrentFreak, a website that covers piracy, described the leak as “without doubt one of the most prominent leaks in TV history”.
The site said illegal downloads will most likely double if the totals of streaming and direct download sites are added to downloads from public torrent sites.
"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," said HBO in a statement.
The leak appears to have originated from copies of Game of Thrones sent out to recipients approved by HBO, according to Jeff Cusson, a spokesman for the network in New York. HBO sent out copies to media outlets ahead of the new season.
“We’re actively assessing how this breach occurred,” Mr Cusson said.
Game of Thrones ranks as one of the world's most-pirated shows, according to TorrentFreak.
Despite the leak, Dan Rayburn, who runs the StreamingMedia blog believes viewers would prefer to view the show in higher definition than the lower standard definition of the pirated episodes, said .
Most viewers would still rather see an effects-laden show like Game of Thrones on a big screen and in high definition, he said. "The leak will impact some viewers, but just how many we have no way to know," Mr Rayburn said.
“Many want to see it in better quality.” The episode, which aired at 9pm New York time on Sunday, picked up just after the murder of patriarch Tywin Lannister.
Game of Thrones has gained such a cult following since it began airing in 2011 that a filming site on Croatia's southern coast is attracting visitors.
HBO won’t release TV ratings until later in the week. The premiere of season four drew 6.6 million live viewers, and millions more in encore airings.