Oscar-winning film producer Harvey Weinstein has been warned not to pursue his current high-profile campaign against the Motion Picture Association of America (MPAA) over its prohibitive rating for the documentary Bully.

Weinstein and his brother Bob, who have been instrumental in bringing the anti-bullying film to the big screen through The Weinstein Company, are unhappy the documentary has been handed an R rating, which means many of its target audience will not be able to see it in cinemas, and have threatened to boycott the MPAA's rating process altogether over the decision.

However, the National Association of Theatre Owners (NATO), which represents cinemas, said earlier this week that any such move might result in all Weinstein Company films being handed an automatic NC-17 rating in future.

"If you decide to withdraw our support and participation in the rating system and begin to release movies without ratings, I will have no choice but to encourage my theatre-owner members to treat unrated movies from the Weinstein Co in the same manner as they treat unrated movies from anyone else," NATO president John Fithian wrote in a letter to the Weinsteins.

He added: "As a father of a nine-year-old child, I am personally grateful that TWC has addressed the important issue of bullying in such a powerful documentary. Yet were the MPAA and NATO to waive the ratings rules whenever we believed that a particular movie had merit, or was somehow more important than other movies, we would no longer be neutral parties applying consistent standards, but rather censors of content based on personal mores."

The Weinsteins hit back with a statement of their own which referenced a recent high-school shooting in suburban Cleveland in which a student who was allegedly the victim of bullying shot dead three other teenagers. "As a company we have the utmost respect for NATO, but to suggest that the film Bully could ever be treated like an NC-17 film is completely unconscionable, not to mention unreasonable," they wrote. "In light of the tragedy that occurred yesterday in Ohio, we feel now is the time for the bullying epidemic to take centre stage, we need to demand our community take action."

More than 75,000 people have signed an online petition urging the MPAA to overturn the R rating handed to Lee Hirsch's film, a decision which was made on the grounds of language. There are said to be six instances of the word "fuck" being used in the documentary, which is due to open on 23 March in the US.

While the MPAA ratings are in theory not legally binding, they are firmly established in the US and any breakdown of the current voluntary system would risk the appointment of a more prohibitive federal or state-appointed censor, something neither studios, film-makers or cinema owners would want.