This is not a kiss but a genetic transfer ... guess Who said that?

Doctor Who and his new assistant enjoy a lingering onscreen kiss in the new series of the hit BBC show.

The Doctor, played by David Tennant, kisses Martha Jones, newcomer Freema Agyeman, on the lips but then says: "It means nothing, it was just a genetic transfer."

The kiss is in the first episode of the third series, which had a special screening at the Mayfair hotel. In later episodes, the Doctor gets married - while a jealous Martha looks on. He is believed to wed a character played by comedy actress Jessica Stevenson in a "flashback".

In a comment that will get fans guessing, Tennant said: "Yes. He gets married. Well, he doesn't not. Actually, it's quite difficult to answer that question truthfully."

Londoner and former Crossroads actress Agyeman, 27, is making her debut as the Time Lord's latest assistant, replacing Rose Tyler who was played by Billie Piper.

The action begins with Martha, a trainee doctor, finding herself - and her hospital - transported to the moon. Agyeman said of her role: "Hands on heart, I didn't have Billie's character in mind when I took this on but looking at it now I can see there are similarities. But the Doctor is always going to look for the same sort of thing in an assistant. There's got to be a feistiness, you're going to get a happy-go-lucky, bolshy kind of person."

BBC head of fiction Jane Tranter said of Agyeman: "She comes bursting on to our screens with intelligence, fearlessness, charisma and beauty. She tackles the Doctor like no other companion has done before."

Writer and executive producer Russell T Davies confirmed that there would be a fourth series, but refused to answer speculation that Tennant will bow out.

Jonathan Ross, Dawn French and Catherine Tatewere at the screening.