The show's official Twitter account urged fans to refrain from sharing details of the episode "The Final Problem" before it airs tomorrow. Images that appeared to be from it were being shared today on social media.
The show stars Benedict Cumberbatch in a modern take on Sherlock Holmes and Martin Freeman as his sidekick Watson.
The leak was announced two days after a preview screening of the "Sherlock" fourth season finale at the British Film Institute, where audience members were asked not to give away spoilers.