Now Jamie Oliver seeks to save our bacon

Animal rights campaigners are trying to close a loophole on "free-range" meat ahead of an expected surge in demand from shoppers jolted by a new campaign by the TV chef Jamie Oliver.

In a Channel 4 special this month, Oliver will investigate how pigs are reared in the UK and on the Continent, where factory farms supply cheaper meat to British supermarkets. It is expected to show farmers castrating pigs without anaesthetic and preventing pregnant animals from moving – both practices eschewed by UK farmers. Oliver will also look at how shoppers are confused by an array of labels describing the production of pork, ham and bacon.

Unlike free-range chicken, there is no legal definition of free-range pork, meaning that "free-range" packs could come from pigs weaned outside but kept indoors for the majority of their lives. Sainsbury's, Asda and Morrisons avoid stocking free-range pig meat because of the absence of standards.

Tesco does stock "free-range pork", but would not tell The Independent on Sunday how much time its pigs spent outdoors. Britain's biggest retailer would only say: "Our free-range pigs are born, reared and finished in paddocks with metal arcs or insulated tents for shelter."

The RSPCA launches a campaign, Rooting for Pigs, tomorrow in an attempt to galvanise public opinion on pigs, which it describes as "highly intelligent and inquisitive animals".

The charity said: "They are often rated as the fourth most intelligent animal, behind primates, dolphins and elephants, and there are concerns that many pigs are raised in conditions that the public would think horrifying if applied to any of these other species."

The RSPCA says it does not have evidence that producers are exploiting the legal ambiguity but wants consumers to be able to respond to its campaign and Oliver's show, called Jamie Saves Our Bacon. "Consumers have the potential to be misled by labels because there's no set way a free-range pig has been reared," said an RSPCA spokeswoman, Calie Woozley.

"I wouldn't say it's deliberately to dupe shoppers, but because the law is unclear there's that potential for consumers to be misled."

The RSPCA is holding talks with the industry and the big four retailers with the intention of unifying farming standards. Supermarkets – which import 62 per cent of pig meat – currently use a variety of terms to describe production. "Outdoor bred" means pigs that have been kept outside for only the first four weeks, while "outdoor reared'"animals should have been able to access the outdoors regularly. Standard pigs are kept indoors, often in barren concrete pens.

MPs on a Defra select committee will next week publish their investigation into the collapse of British pig farming. In October, the Labour MP David Taylor said the committee had received evidence that, while shops had increased prices by 30p per kilo between 2007 and 2008, they had passed on only 1.5p a kilo to producers.

However, the North West Leicestershire MP warned that the public was hypocritical about animal welfare. "If you stand with a clipboard outside any Tesco or Asda and ask people to endorse the sentiment about higher welfare standards for poultry or pig meat, people are pushing you out of the way to sign," he said. "The same people moments later will be dipping into the chiller cabinet and picking the cheapest cut."