Stars of BBC One drama Sherlock appear to have taken a trip through time as Benedict Cumberbatch and Martin Freeman pose in period costume for a forthcoming one-off special.

The BBC has previously confirmed Sherlock will return with a special followed by three more films.

Sherlock producer Sue Vertue, who is married to writer Steven Moffat, suggested that the wheels are now in motion for the consulting detective’s return by tweeting an image of its stars Cumberbatch (who plays Sherlock Holmes) and Freeman (John Watson).

She accompanied the picture with a caption which read: “#221back #Sherlock #notkidding.”

Vertue also posted an image of a script for the programme, written by Moffat and Mark Gatiss, and said there had been a read-through earlier in the day.

She wrote: “This is what we’ve been doing today.”

No date has been given for the film to be finished, but the BBC has previously said filming will begin in January.

And there is no explanation why the pair are in period costume for the programme which has updated Arthur Conan Doyle’s characters to the present day.

Although the two stars are pictured next to each other, the image appears to have been assembled from two separate shots of the stars as a montage.