The BBC is investigating how an episode from the new series of Doctor Who was leaked on the internet.

The 45-minute instalment from the multimillion pound revival, the first new series of the programme in 15 years, is named Rose after the doctor's assistant, played by Billie Piper. Christopher Eccleston plays the most recent incarnation of the Time Lord.

The BBC said the leak was “a significant breach of copyright which is currently under investigation”. A spokeswoman said, “The source of it appears to be connected to our co-production partner.

“We would urge viewers not to spoil their enjoyment and to wait for the finished version which airs at the end of the month.”

The leak appears to be that of the first episode, but it is not clear whether it is the final cut. It begins with the Doctor and Rose being chased by Autons, shop mannequins that have come to life, according to the BBC's entertainment website.

Yesterday Russell T Davies, the writer and executive producer, said Eccleston approached him about playing the Doctor.

Davies told the Radio Times, “He emailed me and said if we were looking for a Doctor Who he'd be interested.

“The first couple of episodes were written before Chris was cast so I'd established a template for what I wanted, which fitted Chris perfectly. That was a happy accident. We both wanted to strip it down, make it more down-to-earth.”

The Doctor, an alien traveller visiting earth who has a history of fighting evil, will be eccentric, yet more serious that most of his predecessors.

Rose Tyler has a job, boyfriend and family who have to come to terms with her new space-hopping life.