Sherlock stars Benedict Cumberbatch and Martin Freeman were the first to know there would be a fourth series of the BBC1 detective drama, says co-creator Steven Moffat at the Cheltenham Literature Festival.– even before himself or the BBC's commissioners.

"We had to inform the BBC that Martin and Benedict had commissioned a new series," said Moffat.

"They signed themselves up. They both announced that they were carrying on – so that’s good. Benedict, at some red carpet event somewhere, said he was carrying on forever. Martin, at another one, said, ‘Yep, series four is happening’."

Fans are still awaiting Sherlock series three almost two years after the season-two cliffhanger finale was first screened, with an official air date yet to be announced. And while Moffat admitted he himself did not know for sure when it would be broadcast, he did say "I would place good money on it being at the very end or the very beginning of the year."