The BBC is trying to establish whether an episode of Sherlock was deliberately leaked from within the offices of a Russian state broadcaster, after last night’s hotly-anticipated series finale was circulated on the internet ahead of transmission.

A Russian-language version of the 90-minute episode, entitled The Final Problem, appeared online on Saturday, featuring a three-second continuity announcement identifying it as having originated from Channel One, which holds the rights to air Sherlock in Russia.

BBC Worldwide, which sells the corporation’s content overseas, said it had “initiated a full investigation” into the leak, which analysts suggested could have been deliberately orchestrated to damage the British broadcaster.