A 36-minute clip of the latest Harry Potter movie has been leaked online ahead of its international release.
Warner Bros said it was "working actively" to remove the video, which it said was "stolen and illegally posted" on file-sharing websites on Tuesday.
"We are vigorously investigating this matter and will prosecute those involved to the full extent of the law," it added in a statement.
Harry Potter and the Deathly Hallows Part 1 opens in the UK on Friday.
It is one of two films to be adapted from the final book in JK Rowling's best-selling series.
It is not yet known how the first 36 minutes of Deathly Hallows Part 1 came to be posted online.
A studio spokesman said its distribution "constitutes a serious breach of copyright violation and theft of Warner Bros property".
"We are working actively to restrict and/or remove copies that may be available," its statement added.
Deathly Hallows Part 1 could have the biggest opening weekend so far for the fantasy film franchise, according to industry experts.
They believe it could pass the $100m(£62.7m) mark in North America in its first three days and take more than $1bn (£627.5m) in global ticket sales.
According to the Box Office Mojo website, the first six Harry Potter films have earned $5.4bn (£3.4bn) at the worldwide box office.
Harry Potter and the Deathly Hallows Part 2 will be released next summer.