In an interview with the Radio Times, Moffat said: “We’ll find out that Matt Smith is actually the 13th Doctor. Although everyone knows that the Doctor can only regenerate 12 times.”
“The 12 regenerations limit is a central part of Doctor Who mythology – science fiction is all about rules, you can’t just casually break them.”
“So if the Doctor can never change again, what’s Peter Capaldi doing in the Christmas Special?”
The Christmas Special, ‘The Time of the Doctor’ will air on Christmas Day at 7:30pm on BBC One.
"It’s Nearly Time" by our graphic designer Aaron Gittoes.
Steven Moffat has revealed he always planned to feature Peter Capaldi in The Day of the Doctor.
"It was the plan from the start - all the Doctors… all the Doctors will fly in to save Gallifrey and change the timeline."
"That meant we should have the new one. I knew there was going to be a new one, so I wrote it not knowing who would be doing that [scene]."
"We did manage to cast somebody where you can use less than half his face, for less than a second and everybody knows who it is!"
"It’s just another treat - I was just trying to throw treats at the screen!"
Doctor Who returns on Christmas day (25 December) on BBC One.