The sixth series will see Matt Smith and Karen Gillan reprise their respective roles as the Doctor and Amy Pond. Writing on Twitter, Moffat asked if anyone knew what "7777" could refer to.
A short while later he confirmed: "Okay. Episode 7 of the new series, is the 777th episode of Doctor Who. "
The writer added: "Oh, and we just started shooting it."
The seventh episode will serve as a mid-series finale, following the decision to split the BBC show's sixth run.
Doctor Who will return in the spring.
> 'Sherlock' writer joins 'Doctor Who'