Sherlock's Benedict Cumberbatch is 'thrilled' to return for the fourth series

Benedict Cumberbatch said he's "thrilled to be back" as filming for the fourth series of Sherlock is under way.

He returns to the popular BBC One show in the leading role, with Martin Freeman as his trusty assistant, Doctor Watson.

Fans will see the famous detective "back on British soil" and Doctor Watson will tackle a new challenge: parenthood.

Benedict said: "I can't wait for everyone to see season four. But you will have to wait ... though not for long ... and it will be worth it."

The series will return with three feature length episodes, the first of which will be directed by Rachel Talalay. Her previous TV credits include Doctor Who, The Flash and The Wind In The Willows.

In a joint statement, co-creators and writers Steven Moffat and Mark Gatiss said: "Sherlock series four - here we go again!

"Whatever else we do, wherever we all go, all roads lead back to Baker Street - and it always feels like coming home. Ghosts of the past are rising in the lives of Sherlock Holmes and John Watson, bringing adventure, romance and terror in their wake.

"This is the story we've been telling from the beginning. A story about to reach its climax ... "

Mark, who also plays Sherlock's brother Mycroft, posted a teaser video on Twitter of himself holding a director's clapboard, emblazoned with "Sherlock IV" and telling the camera: "We're back. The game is on."

He was recently named best supporting actor for his role in Three Days In The Country at the Olivier Awards, largely considered the most prestigious theatre event in the UK calendar.

The show's executive producer, Sue Vertue, said: "It's taken a while to gather everyone together for series four but I can confidently say I think it will be well worth the wait!"

Fans last caught a glimpse of their favourite detective on New Year's Day when Sherlock: The Abominable Bride aired. It was watched by 11.6 million viewers, making it the highest ever audience share for a Sherlock episode, according to the BBC.

Steven announced in January that he had quit his role as lead writer and executive producer of Doctor Who.

He will be replaced by Broadchurch creator Chris Chibnall, while series 10, scheduled to be broadcast in 2017, will be his last.

No air date for the new series of Sherlock has yet been given.