BBC has launched an investigation into how the final episode of Sherlock season 4 found its way online. Perviy (Channel One), the Russian broadcaster apparently behind the leak, suggests it was hacked. 'The Final Problem', which aired January 15, became one of Sherlock's final problems, after a Russian-language version of the finale was illegally leaked online.

The leak featured a brief continuity announcement linking it to Russia's Channel One, which owns the rights to show the series in the country.

A spokesman for the BBC confirmed: "BBC Worldwide takes breaches or our stringent content security protocols very seriously and we have initiated a full investigation into how this leak occurred."

Channel One Russia has claimed in a statement that an "external third party" may have caused the episode to emerge online.

"Channel One Russia has been cooperating with BBC Worldwide from the moment we were alerted to the appearance of the leaked episode and we are doing everything in our power to determine the source of the breach," the network said. "Initial investigations suggest our content system was compromised by an unauthorized external third party.

"Channel One Russia will share the results of our investigation with the media as soon as it has concluded. Channel One Russia would like to apologize for any inconvenience this leak has caused."