Russia's Channel One has blamed the leak of the Sherlock season four finale on one of its employees, who it says "non-maliciously broke" the company's security protocols.

An investigation was launched by the BBC after a Russian language version of the show, featuring a three-second continuity announcement that linked it to Channel One, emerged online a day before the episode was due to air.

Russian communications watchdog Roskomnadzor demanded that all pirated versions of the episode be taken down, while the show's creators urged fans not to watch the leaked version.

A BBC source reportedly said that the leak was "more than an accident" while an official spokesman said: "BBC Worldwide takes breaches of our stringent content security protocols very seriously and we have initiated a full investigation into how this leak occcured."

However a statement from the network on Friday blamed a single employee, spoke of a "gross dereliction of duty" and promised that content security had been "completely revised".

"We regret to confirm that one of our employees non-maliciously broke our strict security protocols," Channel One said. "As a result of this gross dereliction of duty, the file leaked online.

Sherlock season 4 in pictures Show all 7 1 /7 Sherlock season 4 in pictures Sherlock season 4 in pictures Mark Gatiss and Benedict Cumberbatch in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: Mycroft Holmes (MARK GATISS) and Sherlock Holmes (BENEDICT CUMBERBATCH) Copyright: Hartswood Films 2016. Sherlock season 4 in pictures Una Stubbs as Mrs Hudson in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: Mrs Hudson (UNA STUBBS) Copyright: Hartswood Films 2016 Sherlock season 4 in pictures Mark Gatiss in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: Mycroft Holmes (MARK GATISS) Copyright: Hartswood Films 2016. Sherlock season 4 in pictures Toby Jones in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: Culverton Smith (TOBY JONES) Copyright: Hartswood Films 2016. Sherlock season 4 in pictures Martin Freeman in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: John Watson (MARTIN FREEMAN) Copyright: Hartswood Films 2016. Sherlock season 4 in pictures Martin Freeman, Amanda Abbington, and Benedict Cumberbatch in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: Behind-the-scenes filming 'Sherlock' Season 4. John Watson (MARTIN FREEMAN), Mary Watson (AMANDA ABBINGTON) and Sherlock Holmes (BENEDICT CUMBERBATCH) Copyright: Hartswood Films 2016. Sherlock season 4 in pictures Amanda Abbington, Benedict Cumberbatch, and Martin Freeman in Sherlock Sherlock, Season 4 premieres January 1, 2017 on MASTERPIECE onÂ PBS. Picture shows: Mary Watson (AMANDA ABBINGTON), Sherlock Holmes (BENEDICT CUMBERBATCH) and John Watson (MARTIN FREEMAN) Hartswood Films 2016

"We would like to reassure our partners and viewers, that as a result of this incident, we have completely revised our content security procedures to ensure that this does not happen again, and we apologise to Hartswood Films, BBC Worldwide and Sherlock fans everywhere who have been impacted by this leak."