The BBC is trying to work out whether the final episode of its flagship Sherlock series was deliberately leaked by Russia’s state broadcaster.

The British corporation aired the third and final instalment, The Final Problem, of one of its most internationally successful creations on Sunday night.

But a Russian language version, featuring a three-second continuity announcement linking it to the country’s Channel One, emerged online the previous day.

Channel One owns the rights to show Sherlock in Russia.

A BBC source reportedly said the leak was “more than an accident” and a 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 occurred.”

Sherlock: Season 4 finale trailer

Sherlock, which premiered in 2010, and features Benedict Cumberbatch, is streamed in more than 180 territories and the third series was the UK’s most watched drama of all time.

Its 2016 New Year's Day special, The Abominable Bride, was apparently the biggest overseas export of the last financial year after selling to 216 territories.

This week’s leak mirrors a 2014 incident when unfinished footage from Doctor Who, whose writer Steven Moffat is also the producer-writer for Sherlock, trickled out of the BBC Worldwide Miami office.

Sherlock- series 4 trailer

The Kremlin is believed to have been angered by a recent decision to expand the BBC World Service’s Russian language output.

It is also understood Moscow threatened to freeze BBC finances in Russia and report Britain to watchdogs after Royal Bank of Scotland (RBS) subsidiary NatWest considered closing the account of state-funded Russia Today.

A Russian foreign ministry spokesman reportedly said the NatWest threat “reeked of” BBC intervention.

Russia has repeatedly been blamed for a string of cyber-attacks, including the hacking of the US election.

Ben Nimmo, an information defence fellow at the Atlantic Council think-thank, said: “As Sherlock Holmes said himself, ‘When you’ve eliminated the impossible, whatever remains, however improbable, must be the truth’.

“There appears to be no profit motive, no benefit to the broadcaster from doing this. What remains is a political motive. The most obvious explanation is that this is punitive.”

But Russia analyst and senior economist at HIS Global Insight Lilit Gevorgyan told The Telegraph: “It could be a simple case of poorly executed intellectual property handling.”

The show's producer Sue Vertue announced the leak on Saturday and tweeted: "Russian version of #Sherlock TFP has been illegally uploaded. Please don't share it. You've done so well keeping it spoiler free. Nearly there."