Homeland (Series 3)

It's the hit show in which CIA agent Carrie Matheson and Sgt Nicolas Brody battle al-Qaida terrorist sleeper cells deep within suburban America.

But the third series of Homeland may have fallen victim to its own "enemy within" after a pirate copy was leaked online, a month before it was due to air, by an insider.

The leaked copy was downloaded an estimated 100,000 times within hours of it being made available by online pirates via BitTorrent file-sharing sites.

"The source of the leak is likely connected to a post-production studio," said a source.

The show's producer 20th Century Fox said its anti-piracy unit was investigating. The corrupted copy is missing the opening credits and some special effects, while an actor's voice has been dubbed in one scene.

The third season of the award-winning show will be screened in the UK from October.

The leaked episode was said by some to be one of the preview copies handed to reviewers at a Television Critics Association event in July.

Some observers said the leak was a publicity stunt to generate extra publicity, according to Showtime channel.

Homeland is one of the most pirated shows on the internet. The first two series were illegally downloaded almost 2.4 million times.

"While the early sampling could conceivably eat into the audience that might otherwise subscribe to Showtime, another school of thought dictates that such a sneak peek has promotional power, spreading the word in advance of the premiere," said a columnist for Variety magazine.

Homeland first hit TV screens in 2011. It has been nominated for 11 awards at this year's Emmys, including outstanding drama series, and best lead actor and actress for its stars Claire Danes and Damian Lewis.

"Showtime hasn't officially responded to the leak but it could be that they're not too bothered by the free publicity," the Torrent Freak website noted.

"Several TV insiders have gone on the record stating that piracy may actually help TV shows, because of the word-of-mouth promotion it generates."