Sherlock faces down his demons in new trailer for series four

Co-creator Steven Moffat previously hinted that this would be the darkest season yet

Toby Jones makes an appearance as Sherlock villain Culverton Smith in a new trailer for series four, and it seems that he has the titular hero right where he wants him.

"The roads we walk have demons beneath. And yours have been waiting for a very long time," a voiceover says at the beginning.

"What's the very worst thing you can do, to your very best friends?" Culverton asks. "Tell them your worst secret."

Co-creator Steven Moffat previously hinted that this would be the darkest season yet, and it looks like that will be the case.

Speaking at a Bafta celebration of the show last week, he said that it "goes to a darker place than I think we've gone before".

Perhaps the most shocking thing of the short trailer is the "I love you" that Sherlock utters at the end - considering the character rarely reveals his emotions, fans will assume that something is up.

Sherlock star Benedict Cumberbatch has suggested that season four could be the last. The first episode will air on New Year's Day at 9am on BBC1.

The season finale, titled 'Sherlock: The Final Problem', is screening at 300 cinemas - including at branches of Odeon, Cineword, Vue and Picture House, around the UK and Ireland "with exclusive content", at the same time that it airs on BBC1 on Sunday 15 January.

'The Final Problem' is the name of Sir Arthur Conan Doyle's short story where Holmes was meant to meet his end, plunging from the top of the Reichenbach Falls with arch enemy Moriarty. But fans were so devastated that Conan Doyle decided to bring him back.

Fans will know that this plot was already used in a previous series, so that Final Problem is unlikely to have anything to do with that part of the story.