Russia's Channel One was set to broadcast the final episode of the fourth series of the popular detective drama starring Benedict Cumberbatch on Monday just after midnight Moscow time (2100 GMT), simultaneously with Britain.
But the full episode professionally dubbed into Russian by the channel was leaked online Saturday and swiftly copied across numerous sites.
"According to preliminary findings, the cause was a hacker attack," Channel One spokeswoman Larisa Krymova said in a statement sent to AFP.
She said that the channel "has been in close contact with the BBC from the moment it learnt of the leak and is carrying out an investigation to identify the source of the material uploaded onto the Internet."
Channel One "will be ready to share full information on the incident with colleagues after the investigation is completed," Krymova said.
The BBC said Sunday that it had launched a full investigation, the Telegraph website reported, also citing a source at the corporation as claiming that the leak was "more than an accident."
Russia's most popular social networking site VK.com responded to a wave of posts discussing Sherlock over the weekend by adding an automatic warning saying "Spoilers: Sherlock" to every message mentioning the show.
On Russian-language Twitter, hashtags and jokes about the leak were trending over the weekend. "That moment when the Russians have watched your show before you," wrote one Twitter user, @zaz1995.
Prime Minister Dmitry Medvedev's official Twitter account posted its first ever "like" on a comment about the show.
Russia's love affair with Sherlock Holmes was fired by a televised version with Soviet actors filmed in the Baltic state of Latvia that aired in the 1980s and is still popular today.
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