A "work print copy" of Star Wars: Episode III - Revenge of the Sith with a time code, rather than the finished version, appeared online on Thursday.

A tracker site showed more than 16,000 people were downloading the film.

Makers Lucasfilm Ltd and the Motion Picture Association of America (MPAA) were not available for comment.

The leak comes as Star Wars fans enjoyed the widest simultaneous global movie release of all time.

Worldwide release

The film, the last instalment in George Lucas' six-film, 27-year saga, will be in cinemas in more than 100 countries by Friday, according to industry publication Screen Daily.

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In the US and UK, fans queued to be the first to see it at screenings at one minute past midnight on Thursday.

Some 18,700 prints are in cinemas around the world - including 9,700 distributed to 3,700 North American theatres.

The film, which completes the story of how Darth Vader was born, came out in a handful of countries on Wednesday with most of the world getting to see it on Thursday.

More countries follow on Friday. Japan and South Korea are the only major markets where fans will have to wait longer, Screen Daily said.

Fans dressed as the film's characters as they queued for hours - or even days and weeks - for the first screenings.

A US consulting firm has calculated the film's release could cost $627m (£342m) in lost productivity.

The previous five Star Wars films have made a total of $3.8bn (£2bn) at box offices worldwide.

Piracy clampdown

Meanwhile, DVD pirates are preparing to flood the market with illegal copies of the film, UK authorities said.

Sleeves for pirate copies of the film and discs with director George Lucas' introduction have been seized in London.

The Star Wars leak comes after the first episode of the latest Doctor Who series appeared on the internet on March 7, three weeks before the series started on BBC One.

The BBC did not provide details about the individual responsible for the leak who was sacked two weeks later, saying only that the person worked for a "third-party company in Canada".