The BBC is investigating how an episode of the new Doctor Who series ended up on the internet three weeks before the show is due to begin on BBC One.
Christopher Eccleston is the Doctor with Billie Piper as his assistant
A show spokesperson said the leak was a "significant breach of copyright".
"We would urge viewers not to spoil their enjoyment and to wait for the finished version, which airs at the end of the month," a statement said.
Christopher Eccleston plays the Doctor in the first new series since the sci-fi favourite was cancelled in 1989.
A 45-minute episode called Rose appeared on the internet on Monday. Rose is the name of the Doctor's assistant, played by pop singer Billie Piper.
Eccleston volunteered for the role by e-mailing the show's writer
"The source of it appears to be connected to our co-production partner," the BBC statement said. The partner is the Canadian Broadcasting Corporation (CBC).
A CBC spokesperson said: "We are looking into it. That's all I can say at this point because we don't know exactly what happened. It certainly wasn't done intentionally."
The episode appears to be the series' first instalment. The new series is hotly anticipated by fans, who have remained avid followers since the end of the last series more than 15 years ago.
The show has since been resurrected for a one-off TV movie and in the form of animated online adventures. In 2003, it was voted the show people would most like to see back on TV.
It has also been revealed that Eccleston, who appeared in the film Shallow Grave and TV's Cracker, e-mailed writer and executive producer Russell T Davies to ask for the lead role.