By Rory Cellan-Jones
Technology correspondent, BBC News
Top Gear is popular with file-sharers in the US
Millions of television viewers are now using file-sharing services to access free and unauthorised copies of programmes, research has revealed.
US drama Heroes was the most popular illegal download this year, according to research firm Big Champagne.
About 55 million people downloaded the show, while 51 million chose to access Lost, the second most popular show.
Visits to leading "torrent" sites, which index video and music files, have also nearly doubled in the last year.
The proportion of file-sharing involving films and television rather than music is continuing to rise, the research shows.
"Millions of television viewers now access free, unauthorised versions of favourite shows at least some of the time," says Eric Garland, chief executive of Big Champagne.
"This is a socially acceptable form of casual piracy - and it is replacing viewing hours."
All of the programmes in the top 10 were American, but the survey also examined unauthorised downloads of popular BBC show Top Gear.
Most Popular Film Torrents
The Curious Case of Benjamin Button; 13, 133, 137
Yes Man; 13,038,364
Fast and Furious; 10,613,668
Gran Torino; 9,880,700
Marley and Me; 9,099,219
Slumdog Millionaire; 8,840,884
During the most recent series, the figures show around 300,000 downloads of each episode in the days immediately following their broadcast in the UK.
But the UK accounted for just 4% of the download activity, with 47% coming from the United States.
Big Champagne says Top Gear has been among the most pirated television programmes internationally.
The series appears on BBC America some time after it is shown in the UK, and it appears that some American fans are eager to download it before it is available legally.
The research also looks at unauthorised film downloads and shows they are getting lower audiences than those for TV programmes.
Most Popular TV Torrents
Prison Break; 29,283,591
Desperate Housewives; 21,378,412
Grey's Anatomy; 19,916,775
Gossip Girl; 19,706,870
Top of the chart was Watchmen, downloaded nearly 17 million times, followed by The Curious Case of Benjamin Button, with 13 million.
The Oscar-winning Slumdog Millionaire, a relatively low budget film compared with the two Hollywood blockbusters, was viewed by nearly 9 million unauthorised downloaders.
The research will be presented on Saturday at the Edinburgh Television Festival in a session on what television can learn from the music industry's experience with online piracy.
Mr Garland says there are major differences between the two industries and the impact on television may not be as severe as some TV executives fear: "We may see a lot of disruption but it is premature to say 'we're next'."
"The effect on the business is going to be very different."
Big Champagne's research also shows that the rate of piracy for live events, such as sport or talent shows, is much lower than that for popular drama series.