LONDON Actor David Tennant said on Monday any actor could replace him as cult television time traveller Doctor Who, even former co-star Billie Piper.
Tennant, 37, announced last week he would be quitting the famous role, sparking immediate speculation about his successor.
Bookmaker William Hill has made 44-year-old David Morrissey, who starred in BBC dramas "State of Play" and "Blackpool" its favourite to get the job, while the Daily Mirror is tipping former Eastenders actor Tom Ellis.
Tennant said Morrissey would appear with him in the Christmas edition of the show, but added that neither he nor drama bosses on the sixth floor of the BBC's west London studios knew who would become the 11th incarnation of the Time Lord.
"I genuinely don't know," he told BBC TV's Breakfast programme. "The last I heard, nobody in floor 6 had any idea either.
"So I think we are still at the stage where there are no secrets to be kept: all doors are open."
He said the nature of the role meant any actor -- male or female -- could become the next occupant of the Tardis. "I actually think it's one of those parts that any actor could bring something very valid to, because he can be anything and because it's a sort of blank canvas every time.
"In fact, the difference is the virtue with each doctor, it's not like recasting Tarzan, where you have to got someone who looks good in a loin cloth. It can sort of be anything."
Tennant said even 26-year-old Piper, who co-starred as the Doctor's assistant Rose Tyler and has recently given birth to a baby boy, could be a contender for the character.
"Why not? She's busy breast-feeding at the moment but I'm sure she'll bounce back before too long," he said.
Scottish-born Tennant has become one of Britain's most popular actors since taking on the part in 2005, and will remain in place for five more special shows broadcast this year and next.
Recently he has been concentrating on stage work, appearing in a critically acclaimed production of Hamlet which moves for a sold-out season in London later this year.
Tennant also returns to the small screen this month in a BBC drama "Einstein and Eddington," where he plays a British astrophysicist who was an early supporter of Einstein's theory of relativity.
(Editing by Steve Addison)