LONDON-The series final of One’s Sherlock drew its lowest-ever overnight UK ratings. Around 5.9 tuned in to watch Benedict Cumberbatch in The Final Problem, down from the 8.1m who watched the first episode on New Year’s Day. However, viewing figures will be higher when on-demand services are taken into account.

The BBC says it is investigating how the episode was leaked online before it was broadcast on TV. The last instalment of the fourth series was screened on BBC One on Sunday evening hours after a Russian language version emerged online. It featured a brief continuity announcement linking it to Russia’s Channel One, which owns the rights to show Sherlock in Russia.

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

The series was the fourth outing for Sir Arthur Conan Doyle’s famous detective, played by Cumberbatch, and starring Martin Freeman as Dr John Watson.

Sherlock’s nemesis Moriarty, played by Andrew Scott, made his return as part of a flashback sequence which saw him team up with Sherlock’s secret sister, Eurus, before he died.

The pair enticed the detective, Dr Watson and Sherlock’s brother Mycroft into a series of games. It also featured a cameo appearance by singer Paul Weller.

Writer Steven Moffat has fuelled speculation that The Final Problem could be the last ever Sherlock.

He said: “We’re not planning it to be (the final), but it might be. We could end it there. We couldn’t have ended it on any of the previous series because they always ended in walloping great cliff-hangers, usually by Andrew.”

The episode received mixed reviews from the critics.

Michael Hogan wrote in the Daily Telegraph “if this was the last-ever episode, which it surely won’t be, it worked well as a sign-off”.

“Last night’s episode showcased all the elements that have made this modern-day reinvention such a hit - as well as those that have proved divisive.”

He gave it five stars and called it “an exhilarating thrill-ride”.

But Christopher Stevens in The Daily Mail gave it no stars and said that the episode was “an abject, flailing, noxious mess”, “shockingly bad” and “self-indulgent rubbish”.

“If you’ve woken up this morning with the angry feeling that you were robbed of an evening’s entertainment, I share your sense of betrayal. Sherlock was, quite simply, the most nonsensical, verbose piece of television I’ve ever sat through,” he wrote.

More than 11 million viewers watched the episode broadcast on New Year’s Day once consolidated figures were taken into account, making it the biggest audience of the festive period.