FREEMA Agyeman, who played femme fatale Lola in the ITV soap Crossroads, is to take over from Billie Piper as Doctor Who's sidekick.

She will play Martha Jones, starting next year, after Piper, 23, bows out as Doctor Who star David Tennant's assistant, Rose, on Saturday, during Doomsday, the final episode of the current BBC1 series.

Agyeman, 27, from London, was spotted after auditioning for the smaller role of alien expert Adeola. She won the part but the character suffered a cruel fate at the hands of the Cybermen last Saturday. TV bosses were so impressed by Agyeman that they called her back after former pop star Piper announced she was quitting the sci-fi show. She has had roles in The Bill, Silent Witness and Casualty.

Writer and executive director Russell T Davies said: "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."

He added: "David and Freema are terrific together, and we're delighted. The first scripts have been written, and Martha's a brilliant addition to the Tardis crew, the perfect foil for the Doctor."

Agyeman said keeping the news to herself had been "driving her mad".

She said: "Auditioning with David in secret down in Cardiff was unbelievable, but I never in my wildest dreams thought I'd become the new companion. It still hasn't quite sunk in.

"I'm sure it will slam home first day on set when I'm gazing at David Tennant."

Tennant, 35, said: "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."

Filming for the next series, in which the pair meet Shakespeare, begins in three weeks.