The new Doctor Who will be unveiled live on television this weekend amid tight security in a bid to prevent their identity leaking out.
Sources close to the show said only 10 people know the name of the new doctor, who will replace Matt Smith, and they are using the codename Houdini to try and keep it a secret.
Bookmakers have installed The Thick of It star Peter Capaldi as 6/4 favourite.
The Scottish actor, famous for his role as foul-mouthed Malcolm Tucker in the political comedy, is a life-long fan of the show and wrote a letter to the Radio Times about the Daleks when he was a teenager.
Plans for this Sunday's half-hour show, which goes out at 7pm, were kept secret with a repeat of Celebrity Mastermind put in listings in its place.
The show will be presented by Zoe Ball and feature interviews with Matt and executive producer Steven Moffat as well as celebrity fans and some previous incarnations of the science-fiction hero.
Names mentioned as possible contenders include Daniel Rigby, whose award-winning performance as Eric Morecambe in Eric And Ernie beat Matt to the best actor TV Bafta in 2011, and Cutting It star Ben Daniels.
Bookmakers William Hill have Rigby at 3/1 and Daniels at 5/1 with Rory Kinnear at 8/1.
The firm's spokesman Joe Crilly said: "Peter Capaldi remains the favourite as we await the announcement, but it does not appear that the result is as cut and dry as first thought with Daniel Rigby coming in for some late support."
Steven Moffat said: "The decision is made and the time has come to reveal who's taking over the Tardis. For the last of the Time Lords, the clock is striking 12."
BBC drama controller Ben Stephenson said: "We can't wait to unveil the next Doctor with everyone live on BBC One on Sunday night. Amongst all the speculation and betting, there has been lots of fun and intrigue at work as we've been using the codename Houdini as a decoy.
"It's the biggest secret in showbiz, even those working with the new Doctor on other projects at the moment have no idea they are in the presence of the 12 incarnation."