Image caption Creator Steven Moffat said the special will be a standalone episode

The BBC has released a new image from the upcoming Sherlock special.

The picture features stars Benedict Cumberbatch and Martin Freeman dressed in period costume for the episode, which is set in Victorian times.

Creator Steven Moffat revealed earlier this year the special would be a historical adventure that stood alone from the main series.

Series four of Sherlock is due to return to screens in 2017.

The BBC has not confirmed when the special episode will be broadcast, but it is rumoured to be around Christmas time this year.

Speaking ahead of a Sherlock panel at Comic-Con later, Moffat said the 90-minute episode would be "a different version of the show we're used to".

"It's like we checked the books and discovered we'd set it in the wrong time period by mistake and it's like we're correcting it," he said.

"We're really proud of it, we think it's a real cracker. But that'll be it until series four."

Image caption The BBC teased an image of Benedict Cumberbatch and Martin Freeman in Victorian dress in November

The writer also bemoaned Twitter after quitting the social media platform in 2012.

"I think Twitter is a useful tool for promoting a show, but I think it is the worst possible form of audience research," he said.

"That is not a typical audience member talking. It might be interesting, it might be witty, it might be clever, it might be insightful, but it's not what they're all saying."

He said one of the biggest pitfalls was "mistaking Twitter's reaction for the audience's reaction".

"People on Twitter are having fun, they're making gags... it's like overhearing a pub conversation, when you're at a pub do you tell the truth, or do you try and get attention?"

Moffat added he and co-creator Mark Gatiss had yet to begin writing Sherlock's fourth series.

The third series bowed out last year with the detective appearing to shoot villain Charles Augustus Magnussen, and teased viewers criminal mastermind Moriarty may have cheated death and be set for a return.