Rebecca Eaton, executive producer of Sherlock for US channel PBS, which airs the show in America, says there will never be more than three new episodes each year due to demands on stars Benedict Cumberbatch and Martin Freeman and co-creators Steven Moffat and Mark Gatiss.
"Steven crafts them, and Mark writes some of them. It’s a lot of work, and Steven also does Doctor Who and he worked on Tintin, so there couldn’t be more than three. I think there will only ever be three at a time," said Eaton.
"It’s getting harder and harder to do another season," she told Collider, "not just because Benedict [Cumberbatch, who plays the eponymous detective] and Martin Freeman [his sidekick John Watson] are getting such high profiles, but Steven and Mark are busy and in demand."
Cumberbatch is currently working on the sequel to JJ Abrams's Star Trek movie reboot, as well as voicing two characters in Peter Jackson's Lord of the Rings prequel The Hobbit, in which Freeman plays the title role.
A third series of Sherlock is expected to begin filming later this year for broadcast in 2013.