Filming for the upcoming season of detective drama "Sherlock" is yet to commence, thereby further postponing the date of the season 4 premiere to late 2016 or even early 2017.

It was earlier announced that filming for the fourth season will start in late 2015, but in May producer Steven Moffat said it would not begin until early 2016. One of the reasons for the delay is the busy schedules of its lead actors – Benedict Cumberbatch and Martin Freeman – as well as co-producers Moffat and Mark Gatiss.

"Well, apparently Benedict and Martin are quite popular in the movies these days so it's quite difficult to schedule around them. And obviously Mark and I have our other commitments too, but it's just a matter of scheduling. We're all keen to continue," Moffat told The Telegraph last year.

Although not much about the season 4 plot has been revealed, Moffat said in another interview with Entertainment Weekly that it would be about consequences.

"[Series four] is going to be... I suppose you'd say... consequences. It's consequences. Chickens come to roost. It's dark in some ways - obviously it's great fun and a Sherlock Holmes romp and all that - but there's a sense of things coming back to bite you."

But before season 4 airs, fans will get to see Sherlock and Watson in a one-off Christmas special that will air in December. The special will be set in Victorian times and Sir Arthur Conan Doyle's 1892 story "The Adventure of the Blue Carbuncle" is said to be the inspiration for the one-off episode.

The producers have brought in some changes to the original story to make female characters more prominent.

"In the original stories, none of the women really speak. They just don't talk unless it's specifically germane to the plot," Moffat told Entertainment Weekly. "So it was a challenge for us in terms of what we're going to do with our female characters now that we're back in Victorian times... which sort of became quite central to how we handled it."