The mystery over who will replace David Tennant as Doctor Who deepened yesterday when the BBC gave the first screening of the show’s Christmas special and swore all those who saw it to secrecy.

The corporation said months ago that the hour-long episode, which will be screened at 6pm on Christmas Day, would be called The Next Doctor and that it would feature David Morrissey as a new timelord - prompting the bookmakers to make him favourite to replace Tennant.

But yesterday, at a preview for invited guests, Jane Tranter, the Controller of BBC Fiction, begged those who saw the special not to reveal anything in it that would give away Tennant’s successor.

Referring to Morrissey’s role as a timelord, she said: “For the next 60 minutes you can make of that what you like. You have got to promise not to tell anyone else what to make of that either. You would spoil the surprise.”

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Russell T. Davies, the creative force behind the regeneration of the series, was more forthright. Speaking to reporters after the screening, he said: “Don’t spoil it, all of you. Or you can f*** off.”

An hour later William Hill had replaced Morrissey as favourite to become Tennant’s permanent successor. Paterson Joseph, best known for his role as Mark Corrigan’s boss Johnson in the Channel 4 cult comedy Peep Show, is now favourite to land the part after speculation that the BBC may favour a black actor.

In the special, set in Victorian-era Britain, Tennant teams up with Morrissey to fight the evil Cybermen, led by Dervla Kirwan.

Tennant, who has been sidelined from his West End role in Hamletafter undergoing back surgery, will bow out as a timelord after four Doctor Who specials next year.

Davies said that he was sure that the actor would have recovered in time to begin work on the first instalment, which starts filming on January 19. “The operation that he’s had makes it very flexible,” he said. “It’s a problem that’s been fixed but we’ll have to be very careful with him – I don’t think he will be swinging on a wire on the first day.”

Ms Tranter said: “I think it’s extraordinary, frankly, that David’s back hasn’t gone before because he works so hard. His commitment is extraordinary.” She added that she expected he would soon be saving the Universe as the Doctor and avenging his father as the Prince of Denmark.

Davies fuelled further speculation about the identity of Tennant’s replacement by admitting that he would like to see a woman play the role, and said that he favoured Catherine Zeta-Jones or Lesley Sharp, who worked alongside Morrissey in the BBC drama series Clocking Off.

Asked whether he thought that the role could go to a black actor, he said: “I think the more it’s talked about, the more likely it is to happen.”

But Davies, who will bow out as lead writer and executive producer of Doctor Who at the same time as Tennant departs, said that he had had nothing to do with choosing the next Doctor. “They don’t tell me because I gab,” he said.

His replacement, Steven Moffat, will take a lead role in picking the new Doctor, Davies said. “I think it’ll be a while though because it’s a big deal to cast. Whoever becomes the Doctor has got to take on a whole life. It’s a lot of soul-searching to do.”

Davies said that next year’s special editions would involve filming overseas, as well as in Cardiff, the home of the show. “We’ll be filming abroad,” he said. “It’s going to be exotic. We have got a great guest star.”

He disclosed that the Doctor would be assisted by a number of sidekicks rather than a constant companion.

Other names in the frame to become the next Doctor include James Nesbitt, and David Walliams, the star of Little Britain. He sparked a flurry of bets after admitting that he would accept the role if offered it.

An average of 8.1 million viewers watched the latest series of Doctor Who, the fourth since it returned to television screens in 2005.

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