The show would have starred Bille Piper's character, Rose Tyler

"It was actually commissioned by the controller of BBC One and budgeted," the writer told Doctor Who Magazine.

Davies later decided the programme, Rose Tyler: Earth Defence, was "a spin-off too far" and called it off.

Piper, 23, left Doctor Who earlier this year. She will be replaced by Freema Agyeman, 27, in the next series.

"We hadn't formally approached Billie," said Davies, "although we'd mentioned it to her."

Cold feet

"It was going to be fantastic. We'd have had a lovely budget and done brilliant things with it, maybe one Bank Holiday special a year."

However, the writer said he got cold feet while filming Piper's final appearance as Rose Tyler earlier this year.

A separate spin-off, Torchwood, stars John Barrowman as Captain Jack

"It spoils Doctor Who if we can see Rose... if we see as a concrete fact that her life continues to be as exciting without the Doctor," he said.

Davies added that the decision to abandon the programme cost him "a fortune".

A separate spin-off from the science fiction series is still going ahead, and will be shown on BBC Three this autumn.

Called Torchwood, it focuses on a squad of secret agents facing human and alien enemies, with actor John Barrowman reprising his role as time traveller Captain Jack.

The 13-part series will also be written by Davies, who acts as executive producer for the Doctor Who programmes.

Meanwhile, the third series of Doctor Who is currently being filmed in Wales.