Since David Tennant announced last week that he is to quit as Doctor Who, the race has been on to discover who will replace him.

Current favourites to take over as star of the BBC’s hit science fiction show include Cold Feet actor James Nesbitt, Life on Mars star John Simm, Paterson Joseph and David Morrisey.

But all bets could be off now the outgoing Doctor himself has thrown a new name into the ring: his former intergalactic travelling companion, pop singer turned acting star Billie Piper.

When Tennant was asked on BBC Breakfast today if his one-time assistant could step up to the role, he replied: "Why not?"

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He said: "It's one of those parts that any actor could bring something valid to, because it can be anything and it's a sort of blank canvas every time.

"The fact is that the difference is a virtue with each Doctor, it's not like you're casting Tarzan where you've got to have somebody who looks good in a loin cloth, it can be anything."

If Piper, who has just given birth to her first child, does become the 11th Doctor Who she will be the character’s first female incarnation. Her performance as the Doctor's assistant Rose marked a revival in the fortunes of Piper, who first earned fame as teenage pop singer and the wife of TV personality Chris Evans. Since leaving the Tardis, her starring roles have included Belle in The Secret Diary of a Call Girl.

Tennant who is now playing Hamlet with the Royal Shakespeare Company at Stratford-upon-Avon, also said the decision to leave the series after five years had been a difficult one.

He said: "Sometimes you've got to take a deep breath and make a decision otherwise I could literally be there when I'm 86 with people thinking 'Is he never going?'

"Who knows what might happen in the future and it is one of those parts that sticks with you... the door isn't closed forever, but in a day to day way I'll be handing over the mantle to somebody else."

Tennant is still to film four special episodes of Doctor Who to be shown this year and early in 2010, as well as a Christmas special in 2009. There is no series scheduled for next year because of his commitments with the Royal Shakespeare Company.

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