The British-backed movie Fast Food Nation, a dramatisation of Eric Schlosser's bestselling book of the same name, includes graphic footage shot in a real slaughter house. It claims to shed light on the treatment of animals - and workers - in the fast food industry.

Director Richard Linklater, whose previous work includes School of Rock and Before Sunrise, said burger chains were already reacting to its highly unpalatable findings. "They have hired agencies apparently that are going to be there when we come out as a movie in the States," he said.

"It's a little interesting. I have never made a movie that is suddenly threatening somebody's corporate bottom line."

The film, which received funding from BBC Films and features Bruce Willis, Ethan Hawke, Patricia Arquette and Catalina Sandino Moreno, an Oscar nominee for Maria Full of Grace, details the workings of a fictional burger brand called Mickeys. When faeces is detected in the chain's burger meat, executive Don (Greg Kinnear) is sent to investigate.

Real-life chains such as McDonald's had nothing to fear from the film, Linklater said. But actors who played Mexican immigrants working in a slaughterhouse in the film admitted they were so appalled by the experience they had given up burgers. Ana Claudia Talancon, one of the actors, said: "Just the smell of being in there and the whole process was pretty bad." Linklater said the slaughter scenes were "tough imagery" but added that the film showed "a reality everyone needs to know. We've become very separated, divorced from where everything comes from."

Schlosser, who also wrote the film's screenplay with Linklater, called for the public to be choosier about where they ate: "By refusing to go to the fast-food chains, you're sending a message that animals shouldn't be treated that way, and workers shouldn't be treated that way," he said.

Fast Food Nation does not provide an easy answer. Ethan Hawke said people were pacified by hopeful endings, whereas he and other cast members hoped the film's conclusion might instead spark protest. "Righteous anger can fuel people and provoke change," Hawke said.

Jeremy Thomas, the British co-producer with Malcolm Maclaren, said: "Every newspaper I read, you see that there are stories not only about the food chain but about the labour forces and the movement of people."

Elsewhere in Cannes, Pedro Almodovar, the Spanish director whose new film Volver has proved the most successful Spanish film ever produced on home territory, said he would still like to direct an English-language film - but would not make it in Hollywood.

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Unveiling Volver with his long-time collaborator and star Penelope Cruz, he said: "I think I probably won't shoot a film in English in Hollywood. I could do it somewhere else in the world where the production system is less of a straitjacket," he said.