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Jo Martin, who will play the second female Time Lord, is the real thing, according to Doctor Who show-runner Chris Chibnall.

Fans were astounded last night when a new Doctor arrived to compete with Jodie Whittaker's version, even bringing her own Tardis.

Chibnall said that Martin, who becomes the first black actor in the iconic role, is not playing a fake or phoney version.

“The important thing to say is – she is definitively the Doctor,” he explained. “There's not a sort of parallel universe going on, there's no tricks.

"Jo Martin is the Doctor, that's why we gave her the credit at the end which all new Doctors have the first time you see them. John Hurt got that credit.”

(Image: BBC)

It is not clear where Martin's version, whose human name in the plot is Ruth Clayton, fits into the history of the 13 Doctors.

But fans are guessing that she could possibly sit between Patrick Troughton and Jon Pertwee, because she did not recognise the sonic screwdriver but did have a police box TARDIS, which was invented by William Hartnell's first Doctor.

When she was invited to audition, Martin wasn't told it was for Doctor Who. “We called her in and she read for it – with secret pages that didn't mention that it was the Doctor,” Chibnall laughed.

(Image: BBC / Ben Blackall)

“But she's a massive fan of the show and I think she guessed. I had to ring her up and tell her and she was overwhelmed and thrilled. A lot of her family are fans and she kept that secret brilliantly for so long – nobody knew. We filmed it in the middle of last year.”

He said her casting was the idea of director Nida Mazoor. “She was the best person for the job, it was a great suggestion, she's got a brilliant combination of humour and warmth and wit and softness and steel. She's a really exciting Doctor.”

Chibnall said viewers would find out more about the previously unknown Doctor, who arrived with her own 1970s version of the TARDIS, over time.

(Image: BBC / Ben Blackall)

“There will be answers to some of these mysteries this series. But as ever with Doctor Who, answers often reveal new questions. It's all very deliberate.”

He said that he was not scared by those who said he should not meddle with the so-called Whoniverse.

“We’re telling an ongoing story and there are plenty of things in Doctor Who that change all the time but I’m very aware of the history and very aware of the continuity.

(Image: BBC)

"It’s a delightful thing.

"But stories are ongoing pieces of fiction. They develop and evolve. My job is to be bold with the storytelling and have fun.”

Fans were also thrilled to see Captain Jack Harkness make his return during Fugitive of the Judoon, in which he came face to face with the companions but not with the Doctor herself.

Chibnall hinted he wouldn't be seen again until next year: “You won't see him again this series, I can categorically tell you that.

"But who knows when he might pop up again? We love Captain Jack, we love John Barrowman.

"I hope that won't be his last appearance and that at some point he gets to meet the 13th Doctor."

(Image: BBC)

He added: “Part of the fun of doing Doctor Who is trying to get these treats into the show for people, without anyone knowing, so it feels like a huge present for people for their loyalty.

"We planned it for a long time. We shot those scenes at Clifton cathedral in a special secret day of filming so it was all very cloak and dagger. He had his own code name, we pulled out all the tricks to get it done.”

* Doctor Who returns on Sunday on BBC One.