The FBI are investigating the online leak of an almost finished copy of X-Men Origins: Wolverine, a month before the film's cinema release.
The Hugh Jackman film was downloaded an estimated 100,000 times from file-sharing websites on Tuesday.
20th Century Fox confirmed the copy had now been removed and the FBI informed.
The studio behind Wolverine stated: "The source of the initial leak and any subsequent postings will be prosecuted to the fullest extent of the law."
"The courts have handed down significant criminal sentences for such acts" the studio noted.
'Stolen and incomplete'
Both the FBI and the Motion Picture Association of America are "actively investigating" the crime.
The film is not due for release until 29 April in the UK and 1 May in the US.
Fox called the leaked movie a "stolen, incomplete and early version".
Viewers reported that green screens and wires attached to actors were still visible.
One user wrote on film website aintitcool.com: "The CGI is missing and the movie looks horrible without it.
"Even if you see the workprint you're still going to have to go see it in the theatre to fully experience the full movie with CGI effects fully intact."
Fox confirmed the film "was without many effects and had missing scenes and temporary sound and music."
A statement from the studio said that once the film was discovered online they "immediately contacted the appropriate legal authorities and had it removed".
The studio added that because their content is forensically marked they should be able to trace the person who uploaded it.
A brief caption at the beginning of the film reads 'Rising Sun Pictures' and is dated 2 March, 2009.
Rising Sun is an Australian visual effects company that worked on the film - but there is no evidence to suggest it was involved in the leak.
The film features many special effects, not all of which are in the leaked copy
In a statement, the company's chairman, Tony Clark, said: "From the reports we've had, the stolen material is a work in progress version of the film with many incomplete sections.
"As we worked on individual sequences within the film, neither RSP nor its staff members have ever been in possession of a full-length version, so it would have been impossible for the movie to have been leaked from here.
"It's common practice for work in progress between us and the production to carry vendor watermarks and for these works in progress to be integrated into various edits of the film for screenings, which would explain why our name appears."
The Federation Against Copyright Theft said a leak of this kind was extremely unusual as pre-release security had been tightened in recent years.
Access to a whole film is severely restricted, with post production workers being given small portions of the movie to work on and measures put in place to stop it being copied on to disc.
Spokesperson Eddy Leviten explained: "No-one working on a film in any capacity wants to see their hard work released through illegal means - piracy is not a fun thing."
Although several reviews of the incomplete film have appeared online, Fox was quick to praise the reaction of web users to the leak.
"We are encouraged by the support of fansites condemning piracy and this illegal posting and pointing out that such theft undermines the enormous efforts of the filmmakers and actors and, above all, hurts fans of the film" the studio said.
In 2007 director Eli Roth blamed an online leak of his horror film Hostel: Part II for reducing box office returns.
In an interview with MTV he claimed: "You could buy Hostel: Part II for a quarter in Mexico City. As a result, in a lot of countries where the piracy was bad, they just didn't even release it."
The leak came as a new campaign against film piracy launched in UK cinemas.
In a change from recent years, the adverts move away from the message that piracy is theft.
Audiences will instead see a trailer, fronted by Dr Who actor Noel Clarke, thanking them for paying to come and watch a film.