Marilyn Monroe is faced with an allegation that she appeared in a sex film during her days in the limelight.

The protectors of Marilyn Monroe's image say a collector who wants $500,000 for a short 1940s stag film is committing fraud and violating the actress's intellectual property rights by claiming it shows her having sex when she was underage.

Mikel Barsa, a Spanish events promoter, has been claiming that the black-and-white six-minute film shows the young actress, known then as Norma Jeane Baker, around 1946 or 1947 when she was poor and desperate to break into show business.

On the other hand, experts on Monroe's life, raised their doubts over the alleged sex film of Monroe, comparing the film with known Monroe images. Also, while Barsa shows off various documents he says support his theory, they self-evidently fall short of proof.

Whoever the woman in the film may have been, even alleging that it shows Monroe violates her intellectual property rights and will cost Barsa dearly if he goes ahead with the sale, said Nancy Carlson, a spokeswoman for the brand development and licensing company Authentic Brands Group.

Barsa is inviting legal action for "perpetrating a fraud on the public, violating the Monroe estate's exclusive rights to her image and other claims of intellectual property infringement," Carlson said in an interview.

"Marilyn had no motivation at all to make a porn movie," countered Michelle Morgan, who wrote "Marilyn Monroe: Private and Undisclosed."

"Her friends all agree that she never, ever showed any signs of having earned money in any other way than modeling or working as a real actress," Morgan said.

Monroe "wasn't a prude," said Lois Banner, who wrote "MM-Personal: From the Private Archive of Marilyn Monroe." But Banner also considers it extremely doubtful that financial difficulties would have put her in such a spot. Even at her poorest, in 1947, Monroe had a wealthy couple helping out by providing her with $100 a week, she said.

"She had a very purple sex life at times. But she never made any money being a party girl. She did it to advance her career," Banner said.

And Scott Fortner, a Monroe fan who maintains a blog about her and collects the star's clothing, has sent the AP a series of pictures pointing out differences between the star and the woman in the film, from her teeth to her chin to her ears. Some show that while Monroe had a prominent widow's peak, the hairline of the woman in the sex tape seems straight.

Monroe underwent plastic surgery in 1950 to change her nose and chin, but her widow's peak never changed, Fortner said.

"To me personally, it doesn't even resemble her," Carlson said.