Doctor Who will return for a fourth series in 2008, executive producer and writer Russell T Davies has confirmed.
The third series starts later this month on BBC One
Speaking at the premiere of the third series in London, Davies said: "Series four is officially existing. I'm very excited, but we have known for ages."
But he refused to say whether the star of the hit sci-fi show, David Tennant, would return in the lead role.
The third series, which co-stars Freema Agyeman as new assistant Martha Jones, starts on BBC One on 31 March.
Her character replaces former assistant Rose Tyler, who was written out at the end of the second series after Billie Piper decided to leave the show.
"Billie did a tremendous job," said Agyeman. "I know no-one likes change but the nature of the show is change and hopefully fans will be cool with that and give me a chance."
Davies spoke glowingly of his new star. "I just think people will open their hearts and let her in - they'll love it."
Stars including Jonathan Ross, Jo Whiley and Fame Academy coaches David and Carrie Grant attended the premiere in central London.
The series will feature a two-part story about the Daleks
They saw the first episode of the latest series, which sees Doctor Who share a kiss with his new assistant.
But Tennant, who plays the time-travelling hero, would not talk to reporters about his role in future series.
"Do you know how many times I have been asked that question? Do you know how many times I have answered it?" said the actor.
He was more willing to speak of his affection for Cardiff, where the hit series is filmed.
"Cardiff is a great city and has been very good to us," he said. "It's lovely to film there because everyone is so enthusiastic.
"You try filming in London and everyone's so mean about it and complains that there is a huge camera truck parking in their street.
"Everyone in Cardiff seems to welcome us with open arms. God bless you all, frankly."