The marchioness, the pig farmers and the £140,000 film about muck

An aristocratic activist has infuriated the world's biggest pork producer

Tracy Worcester is a rebel. Aged 14, she was expelled from the £16,000-a-year Hatherop Castle School in Cirencester for slapping a teacher. Thirty-six years later – but showing few of those years thanks to daily runs and a careful diet – she is rebelling again by taking on the world's biggest pig farmer, the £6bn-a-year Smithfield Foods.

For the past five years, the English aristocrat has been researching, filming and narrating Pig Business, a feature-length documentary that examines Smithfield's system of intensive production.

At the core of the Marchioness of Worcester's film is the allegation, denied by Smithfield, that its "lagoon and spray" system, which pumps excreta from its pigs into pools and fields, blights the local environment. Moreover, Pig Business claims, such factory farms are viable only because they do not pay the wider social and environmental costs of their operations.

The marchioness, whose husband, Henry, is heir to the Duke of Beaufort's Badminton estate in Gloucestershire, has funded half the £140,000 film from private income – one of a long line of aristocrats to use their time and money to champion causes close to their hearts.

By taking on Smithfield, though, the granddaughter of the third Earl of Dudley is battling a powerful multinational, which is fighting what it considers an unwarranted attack on its reputation.

Lawyers acting for Smithfield have been in conflict with Channel 4 over the 72-minute documentary. A planned screening in February was cancelled because of legal problems, and two public screenings, at the Barbican Centre in London and the Hay Festival last month, proceeded after scuffles. Now, Channel 4 plans to broadcast the film on More4 this week. According to the marchioness, a star of the 1980s TV show C.A.T.S. Eyes and sister of the actress Rachel Ward, other broadcasters are waiting to screen the documentary, which she plans to put on the internet.

Smithfield says the film's claims are old and irrelevant to its current operations. It has rebutted every allegation and its rebuttals are included in Pig Business. To make the film, the marchioness travelled to Poland, where Smithfield has expanded from the US, and interviewed scientists. In one scene, dressed in jeans and a woolly jumper, she bounds over a factory fence.

After an initial request was turned down, the marchioness interviewed Smithfield's Dennis Treacy, vice-president of corporate affairs. "If you've got 10 million pigs, and the research that I did is that they defecate 10 times as much as a human being, you have got the equivalent of 100 million people defecating every day in a very small area. Surely we have got to understand that there are a hell of a lot of people who are going to be suffering from the stench, and whose fishing and whose swimming in the rivers is going to be jeopardised by that system?" she asked.

Mr Treacy responded: "The system is going to be operated in a way that is not supposed to allow that. As far as things running off into the water, if that happens it's against the law and it's wrong. Overapplying the material to the field, it's against the law. Having it drift over somebody's property is against the law. We go out of our way and go to extraordinary lengths to have a system that doesn't allow that."

Pig Business has attracted the support of the celebrity chefs Jamie Oliver and Hugh Fearnley-Whittingstall and the actor Stephen Fry, who said it was "wonderfully judged".

The marchioness hopes her film will encourage shoppers to buy outdoor-reared and bred pork. "Even in our country, we have large-scale farms where pigs have a very unpleasant life."

True Stories: Pig Business can be seen on Tuesday at 10pm on More4