Image caption Investigators in Wyoming exposed the confined conditions these sows spend most of their lives in.

Animal rights activists in the US have told the BBC that so-called "ag-gag" laws could be copied in other countries including the UK.

The laws are designed to limit undercover investigations on factory farms by campaigning groups.

Around a dozen states have passed or are proposing legislation banning these activities.

Supporters say they are designed to protect the privacy of farmers and agriculture businesses.

Such atrocious public policy sets a dangerous precedent for UK industry, as does the introduction of US-style mega-farms. Ben Williamson, PETA

Large, intensive factory farms have shown significant growth in the US over the past 20 years. Between 2002 and 2007 the total number of livestock on the biggest of these farms grew by more than 20%.

But concerns over the conditions in which cattle, pigs and poultry are raised and slaughtered have prompted many animal welfare groups to mount undercover investigations.

Candid cameras

Because there is no single US federal law that protects animals, welfare investigators have played a significant role in bringing public attention to inhumane practices.

In 2008, a distressing video of staggering cattle secretly recorded at a California slaughter plant led to the biggest meat recall in US history. Last year a recording from a pig farm in Wyoming was used to secure convictions against a number of workers for cruelty.

However in Utah and Iowa the undercover recording of videos like these is now illegal. Several other states including Indiana, Arkansas and Pennsylvania are considering similar laws. Other provisions in these bills require prospective farm employees to disclose any link to animal welfare groups.

These regulations are already having an impact, says Cody Carlson, a former investigator who has documented inhumane activities on farms in several US states.

Image caption Exposure of cruel treatment suffered by cattle at this abbatoir in California lead to the biggest meat recall in US history

"When I applied for a job in Iowa in 2009 and they asked me if I had any affiliations to animal protection groups, I would have had to say yes, I wouldn't have gotten the job and I wouldn't have been able to expose the conditions that raised questions about the egg industry there," he told BBC News.

"It is exactly what these industries want - they want to shut down the conversation that's going on about what is happening with the animals we raise for our food."

In California an animal welfare bill, doesn't appear to be as restrictive as many of the others under consideration. It is supported by the California Cattlemen's Association and it requires anyone who records video or other evidence of cruelty to turn it over to the authorities within 48 hours.

But campaigners are highly sceptical, arguing that requiring the handover of material so quickly would undermine an investigation and prevent the collection of wider evidence of inhumane behaviour.

"They've done a clever twist on it," Charity Kenyon from the Slow Food Movement told BBC News.

"They want to make it look like their concern is animal abuse, but it is all part of the same deal which is to prevent ongoing investigations of the type that ended in the largest recall of beef in the history of the US," she said.

Global impacts

Welfare groups say that the American Legislative Exchange Council is the moving force behind these laws. This group supports conservative causes and promotes legislation to limit the role of government.

They have described animal rights campaigners as terrorists. They support the laws because they believe investigators are threatening the privacy rights of individuals and businesses. However they declined to be interviewed by the BBC for this article.

While the "ag-gag" laws are primarily designed to have impact within the US, many feel they will also have an impact outside the country.

Image caption Prosecutions followed after video evidence of the cramped conditions that chicken were being kept in was made public

"As factory farming spreads like a plague around the world," said Matt Rice from Mercy for Animals, "international agribusiness interests will certainly attempt to import America's ag-gag laws along with its tainted meat and animal abuse.

"The UK and other nations should be on high alert."

In the UK, Peta the animal welfare charity said these US laws were "shameful".

"Such atrocious public policy sets a dangerous precedent for UK industry, as does the introduction of US-style mega-farms," said Peta's Ben Williamson.

"Legislators should instead be passing laws to require cameras in all abattoirs and factory farms in order to catch animal abusers," he said.

Others are concerned that if these laws are passed, consumers around the world will no longer be able to trust that exports of US agricultural products are produced without cruelty.

"A significant amount of meat, dairy and eggs produced on US factory farms goes to foreign countries," Matt Dominquez, from the Humane Society of the US told BBC News.

"Anyone who consumes animal agricultural products imported from the US should be scared. This prevents them from knowing what's going on - it blocks an entire industry from transparency."

Follow Matt on Twitter.