The new series of Doctor Who will be split into two for the first time, with its showrunner, Steven Moffat, promising the show's biggest ever cliffhanger – "an earth-shattering climax".

Next year's 13-part series, the sixth since Doctor Who returned in 2005, will run for seven episodes and then return in the autumn for another six.

Moffat said the "mid-season finale" would be a "game-changing cliffhanger".

He added that next year's Doctor Who would run as two separate series, allowing him to double the number of "event episodes" in the new run, and meant fans would never be more than a few months away from the next instalment of the hit BBC1 show.

"Looking at the next series I thought what this show needs is a big event in the middle," Moffat told the Media Guardian Edinburgh International Television Festival.

"I kept referring to a mid-season finale. So we are going to make it two series – seven episodes at Easter building to an earth-shattering climax, a cliffhanger we could never normally do because it would be too long before it came back. An enormous game-changing cliffhanger that will change everything.

"The wrong expression would be to say we are splitting it in two. We are making it two separate series.

"What I love about this idea is that when kids see Doctor Who go off the air, they will be noticeably taller when it comes back. It's an age for children. With an Easter series, an autumn series and a Christmas special, you are never going to be more than few months from the new series of Doctor Who.

"Tart that I am, we will now have two first nights and two finales, twice as many event episodes as we had before."

Moffat, who was also responsible for BBC1's acclaimed updating of Sherlock Holmes, took over stewardship of Doctor Who from Russell T Davies last year. His first series in charge was acclaimed by viewers and critics alike.

Moffat gave festival delegates a first glimpse of this year's Christmas special, guest-starring Michael Gambon and Katherine Jenkins.

Moffat said he chose Matt Smith as his Doctor on the very first day of casting.

"He has that air about him, he's like a young man built by old men from memory," he added.

He first saw Karen Gillan, who plays the doctor's assistant Amy Pond, on video and was worried that she was "wee and dumpy". When he met her, he said, he was "expecting a beachball and met this giant flame-haired goddess who is slightly too tall for my comfort. Standing next to her when she has heels on, you feel like the sidecar of a motorbike".

Moffat dismissed some press criticism, early in this year's series, that Amy Pond was "too sexy".

"That's like being too funny, too nice, too enjoyable," said Moffat. "I was roaring with laughter at the article in the Daily Mail, which said when did Doctor Who assistants have to be sexy. Since the beginning! There was one in a leather bikini — we're in the nursery compared to that."

Moffat said the show's budget had remained broadly similar despite BBC cuts. But he admitted: "I don't understand numbers. It's a decent budget. I beg for money and more rubber green people and eventually they say OK, you can have a third rubber green person."

He added that he had not considered a female Doctor, which he said would not have been appropriate at this time in the show's history.

"No we didn't. I think about it sometimes and maybe it will happen someday. It wouldn't have been right this time," he said. "A woman can play the part. You have to remember the single most important thing about regeneration is you must convince the audience and the children that's it's not a new man, it's not a different man, it's the same one. It's a bigger ask if you turn him into a woman."

Discussing his future, Moffat said he would not be leaving the show "for a while yet".

Gillan, in the same TV festival sessions, said she was committed to the show for the new series.

She added that filming on the show, which lasts 11 days a fortnight for nine months, meant she was unable to work on any other projects. As for her future, she said she was committed to the new series but was taking it one season at a time.

"I have no idea. You just have to take it series by series, you can't really look beyond that so who knows? I'm having fun right now," she added.

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