A documentary about intensive pig farming due to be screened at the Guardian Hay festival is facing a legal threat from one of the companies it investigates.

Pig Business criticises the practices of the world's largest pork processor, Smithfield Foods, claiming it is responsible for environmental pollution and health problems among residents near its factories.

The film was due to be broadcast on Channel 4 in February but was cancelled because of legal fears. A planned screening at the Frontline Club in London earlier this year was also called off.

On Wednesday London's Barbican centre was forced to delay a screening of the film after Smithfield's lawyers wrote a letter saying that the film was defamatory and included untrue claims. The show went ahead when the filmmaker, Tracy Worcester, signed an indemnity taking personal responsibility for its content.

A spokesman for Smithfield said that the company had never threatened to sue the filmmaker or tried to prevent the film being screened, but had requested that inaccuracies or false allegations be removed.

Pig Business shows the cramped conditions in which pigs are reared, similar to those of battery hens, and claims that waste is inadequately disposed off, leaking into the surrounding environment.

Worcester interviewed people who live near Smithfield farms in the US, where the company started out, who complain of health problems including asthma and digestive illnesses, and fishermen who report that stocks have been destroyed. The film documents the company's move to Poland, where locals claim to experience similar health problems.

Worcester, who spent four years making the film, said: It's crucial that consumers are able to watch this so they know what is being done to their food.

Pig Business is an expose of US industrial pig farming conglomerate Smithfield Foods. It has met with repeated attempts at censorship by the company's lawyers.

Filmmaker Tracy Worcester explains how England's libel laws have helped stall the film's general release, and stopped the world learning more about the environmental realities of intensive livestock rearing.

After a showing of my film, Pig Business, at the Royal Society of Arts on 13th November 2008, Channel 4, which was scheduled to broadcast the film in the New Year, received two letters from lawyers acting for the main focus in the film, Smithfield Foods of America, the world's biggest pig producer and processor.

Fearing the legal might of a $12 billion company threatening to sue, Channel 4 pulled my film just before broadcast on February 3rd 2009. To prepare for the worst, Channel 4 made changes to accord with England's business-friendly libel laws and the UK TV's fairness standards, administered by OFCOM. Despite a further two threatening letters, Channel 4 broadcast the film on its More 4 channel on June 30th.

In the US, the Constitution's First Amendment enshrines free speech as a right. So, if you allege in good faith that a public company is causing harm, as long as the allegations are not made maliciously, the company has to prove that it has not caused the harm. In England however, the burden of proof is reversed. The person making the allegation has to prove their case with scientific analyses, court judgments or credible witnesses.

Not even the tabloids are immune from Smithfield's threatening letters: both The Daily Mail and The Evening Standard have received warning letters for reporting about the film.

On the day of a showing at the Barbican arts centre in London on 27 May 2009, Smithfield's lawyers told the Barbican's management that the film was 'defamatory'. As a result, the audience was made to wait half an hour while the executive producer and myself were told that the showing would only go ahead if we signed a document agreeing to indemnify the Barbican.

Putting it on my website would apparently expose me to Smithfield's litigation in every jurisdiction. So the message will have to be spread guerrilla-style - i.e. below Smithfield's radar. For another nine days, the film will be on Channel 4's web site. It is also available free of charge to anyone who wishes to give a private screening.