A Marilyn Monroe expert said the actress in the film was someone considerably heavier and less feminine than the legendary film star.

"If you look at the chin line, at the jaw line, at the teeth of the woman in the film and compare that to Marilyn you can clearly make some distinctions," said Scott Fortner, who has a sizeable collection of Monroe memorabilia.

The film is not the only copy in existence, nor is it the first pornographic film attributed to the actress.

A 15-minute, 16mm film, reportedly shot in the 1950s and said to show the blonde sex symbol engaged in oral sex with a unidentified man sold for $1.5 million (£920,000) in 2008.

Mr Barsa said his 8mm film was identical to a 16mm reel discovered more than a decade ago and sold to a collector for $1.2 million (£735,000) in 1997. In the 1940s, sex films were often made using side-by-side 16mm and 8mm cameras, since audiences used both formats, he added.

The Spaniard has refused to identify the owner of the films who has sought his help in selling them, but said he planned to auction the unverified film on August 7 at a memorabilia collectors fair in Buenos Aires.

Monroe died in 1962, aged 36.