Five scripts for the new series of Doctor Who have leaked online, seven weeks before the episodes are broadcast.
BBC Worldwide said it was investigating "a security issue around Doctor Who Series Eight where unfinished material has inadvertently been made public".
The statement urged fans not to distribute the spoiler material.
The series - which marks Peter Capaldi's debut as the Doctor - is due to begin on BBC One on 23 August.
Confirming the leak, BBC Worldwide, the corporation's global arm, said: "We deeply regret this and apologise to all the show's fans, the BBC and the cast and crew who have worked tirelessly making the series.
"We would like to make a plea to anyone who might have any of this material and spoilers associated with it not to share it with a wider audience so that everyone can enjoy the show as it should be seen when it launches.
"We know only too well that Doctor Who fans are the best in the world and we thank them for their help with this and their continued loyalty."
The leaked scripts are reported to have originated from a BBC Worldwide office in the US.
The first episode is due to have its premiere at a screening in Cardiff on 7 August, after which the show's stars and executive producer Steven Moffat will take it on a world tour leading up to the first TV broadcast.
The scripts, beginning with episode one, titled Deep Breath, are marked "private and confidential".
Last year, some US fans were sent DVDs containing the series finale three weeks before it had been broadcast.
In May 2011, Moffat criticised those "who call themselves fans" who revealed crucial plot lines ahead of transmission.
"You can imagine how much I hate them," he told BBC Radio 5 live. "I wish they could go and be fans of something else."