Who knows if the Doctor will close door on the Tardis?

Catherine Tate lets slip that her popular co-star David Tennant may be about to call time on his travelling but the BBC is staying tight-lipped

One of the most popular actors to play Doctor Who in the BBC's long-running show may be about to close the door on the Tardis for good, according to his co-star.

The comedian Catherine Tate, who will play the Doctor's assistant Donna in the fourth series, let slip during a radio interview yesterday that the next series could be David Tennant's last. She told the Jonathan Ross programme on Radio 2: "I think it's maybe David's last series."

The BBC has always remained tight-lipped over which actor takes on one of television's most popular roles and refused to comment on Tate's speculation.

When pressed during the interview whether Tennant would still be at the control of the Tardis in 2010, she answered: "Possibly."

Tennant's future as the time-travelling do-gooder in one of the world's longest-running science fiction shows was already in doubt after the BBC announced earlier this year that there would be no series in 2009. Instead there would be three specials, scripted by Russell T Davies. Series four will be broadcast in the spring with a further Christmas special next year.

Tennant has proved to be one of the most popular doctors and the classic series, revived in 2004 after enjoying cult status in the 1960s and 70s, one of the BBC's sure-fire hits. Last year's Christmas show attracted 9.4 million viewers and the BBC has high hopes that this year's show, guest-starring Kylie Minogue, will prove a similar draw.

Unlike others who have played the Doctor and have been typecast by the role, Tennant enjoys a varied career as one of the nation's most popular stage and screen performers. He played the young Casanova in the BBC film in 2005. He also appeared this year in the sitcom Learners.

From July until November next year he will return to the Royal Shakespeare Company to play Hamlet in Stratford-upon-Avon.

There will, however, definitely be a fifth series. BBC fiction controller Jane Tranter said earlier this year: "Doctor Who's journey over the past three series has been one of the most ambitious and exciting that we have had, and I'm delighted to be able to confirm not only three exciting specials for 2009, but a fifth series in 2010."