Jane Tranter, BBC Controller of Drama Commissioning, confirmed last night that a third series of Doctor Who - plus a second Christmas special - has been commissioned.

Tranter also confirmed that Billie Piper, who plays the Doctor's companion Rose Tyler, would feature in every episode of series two including the Christmas special.

Filming of the Christmas special and second series with Casanova star David Tennant - who plays the tenth Doctor - is due to begin next month.

Newcomer Freema Agyeman is confirmed to play Martha Jones, the new companion to Doctor Who, it was announced today by Jane Tranter, BBC Controller of Drama Commissioning.

Doctor Who fans have already met Agyeman as Adeola in episode 12 of the current series of Doctor Who but, as viewers will have seen last Saturday evening, this character suffered a terrible fate at the hands of the Cybermen.

Freema will be playing a brand new character in series three.

Executive producer/writer Russell T Davies says: "The search for a new companion had been underway for some time when I first saw Freema Agyeman, she had come in to audition for the part of Adeola in series two.

"Watching her during filming confirmed what an exciting new talent she was so, under cover of darkness, we called her back in to audition with David for the role of the new companion.

"It was an immediate and sensational combination, and her range, presence and charm blew us all away.

"David and Freema are terrific together, and we're delighted to have chosen her to join the Doctor for more adventures in time and space.

"The first scripts have been written, and Martha's a brilliant addition to the Tardis crew, the perfect foil for the Doctor.

"Martha won't be featured in this year's Christmas special; we've got another surprise in store for that."

Freema Agyeman says: "I've been keeping this secret from my friends for months - it's been driving me mad!

"Auditioning with David in secret down in Cardiff was unbelievable, but I never in my wildest dreams thought I'd actually become the new companion.

"Billie rightfully built up an amazing fan base and she will be missed, but I hope the fans are willing to go on new adventures with me.

"It still hasn't quite sunk in, I'm sure it will slam home first day on set when I'm stood gazing at David Tennant!"

David Tennant adds: "Freema was a joy to work with in episode 12 of the current series. She is not only very talented and very beautiful, she's great fun and I'm delighted she's coming on board the TARDIS full time.

"I can't wait to welcome her into the Who family."

Freema Agyeman is 27 years old and was born and brought up in London.

A new 13-part adventure and Christmas special begins filming in Cardiff in the summer for transmission on BBC ONE.

David Tennant returns to his role as the tenth Time Lord and meets his brand new companion as filming commences on the highly-anticipated third series of Doctor Who for BBC One.

The new series, which commences filming this week, also marks the debut of newcomer Freema Agyeman who steps into Billie Piper's shoes as the Doctor's new companion, medical student Martha Jones.

Freema Agyeman, who will star alongside Tennant in all 13 episodes in series three, says: "I am still pinching myself and can't wait to get started!

"It's been nerve wracking but David has been brilliant in helping me to adjust on my first days on set. I am really looking forward to travelling through time and space with him over the next eight months."

Russell T Davies, writer and Executive Producer, adds: "We were delighted and honoured by the second series' success, and we can promise new thrills, new laughs and some terrifying new aliens.

"The Doctor and Martha are destined to meet William Shakespeare, blood-sucking alien Plasmavores, The Judoon - a clan of galactic stormtroopers - and a sinister intelligence at work in 1930s New York."

Filming of the Christmas episode which guest stars Catherine Tate as a bride who finds her wedding day interrupted by an alien plot to destroy the Earth has already been completed.

The storyline features the return of the Robot Santas and also guest stars Sarah Parish as the evil, alien mastermind.

Returning cast members include John Barrowman who will be making an appearance in the series finale. Guest stars include Anne Reid, Roy Marsden and Don Gilet.

Writers include Russell T Davies, Steven Moffat, Gareth Roberts, Paul Cornell, Helen Raynor, Chris Chibnall and Stephen Greenhorn. The directors include Charles Palmer, Euros Lyn, Graeme Harper and Richard Clarke.

The Producer is Phil Collinson; Executive Producers are Head of Drama, BBC Wales, Julie Gardner and Russell T Davies.