Watch a trailer for the ‘darkest’ series of ‘Sherlock’ yet

It appears to tease the return of one of Sherlock's former enemies

The BBC has released a trailer for series four of Sherlock, which will be the “darkest” yet, according to one of its cast members.

Benedict Cumberbatch and Martin Freeman started shooting the new series in May, joined once again by Amanda Abbington as Mary Morstan, as well as by a new villain played by Toby Jones.

Cumberbatch and Abbington joined the show’s co-creators Steven Moffat and Mark Gatiss for a panel at Comic-Con in San Diego yesterday (July 24), during which Abbington told fans: “It’s really dark. It’s the darkest Steven and Mark have written.”

During the panel, a trailer for series four was unveiled which appears to tease the return of one of Sherlock’s former enemies – watch below.

The BBC has previously revealed that series four will begin with Cumberbatch’s title character “back once more on British soil as Doctor Watson and his wife, Mary, prepare for their biggest ever challenge – becoming parents for the first time”.

Cumberbatch and Freeman returned in a one-off special, Sherlock: The Abominable Bride, on New Year’s Day this year. The special attracted an audience of 11.6 million viewers, making it the most-watched programme of the 2015-16 festive season.

The show’s last series, which also consisted of three episodes, aired in 2014. Cumberbatch and Freeman first played Doyle’s iconic characters in 2010 and each actor has won an Emmy for his performance.