Newcomer Freema Agyeman is to take over from Billie Piper in the third series of Doctor Who, it has been revealed.
Agyeman, 27, will play Martha Jones in the next run of the drama, due to start filming in Cardiff this summer.
Regular viewers saw her in last week's episode where she played Adeola, a minor character who suffered at the hands of the Cybermen.
The current series of Doctor Who finishes on BBC One this Saturday and sees Piper leaving the show.
"When I first saw Freema Agyeman she had come in to audition for the part of Adeola in series two," said executive producer and writer Russell T Davies.
"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 [Tennant] for the role of the new companion.
"It was an immediate and sensational combination."
Agyeman has previously been seen as Lola Wise in the soap Crossroads, and has had small parts in Casualty and The Bill.
The London-born actress was one of several names rumoured for the Doctor Who role, including singer Rachel Stevens and former Eastenders star Michelle Ryan.
"I've been keeping this secret from my friends for months," said Agyeman. "It's been driving me mad."
"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."
Billie Piper has been the Doctor's assistant for two series
Although she has now officially joined the cast, producers say Agyeman will not be featured in this year's Christmas edition of Doctor Who.
"We've got another surprise in store for that," said Davies.
Piper, the Doctor's current companion, revealed she was leaving the programme last month.
The BBC had refused to comment on reports that her character, Rose Tyler, dies in the final episode of the series.
However, last week's episode opened with a voice-over from the character saying: "This is the story of how I died."
Piper, 23, is currently filming a BBC adaptation of Philip Pullman's Victorian thriller The Ruby in the Smoke.
"Billie rightly built up an amazing fan base and she will be missed," said Agyeman, "but I hope the fans are willing to go on new adventures with me."