Chlorine-washed chickens, hormone-fed beef and pork raised on growth-promoting antibiotics. It doesn’t sound very tasty – but this is what could be lining our supermarket shelves after Brexit. Trade deals could allow an influx of meat into Britain from the US, where lower animal welfare standards mean it can be produced more cheaply. A House of Lords report this week warned this could spark a change in our farming. The high animal welfare and environmental standards we have in the UK (set by EU law) could be eroded to allow British meat to compete with cheaper imports.

Last week, Michael Gove, Defra secretary, reassured parliament he was committed to maintaining current standards after Brexit. "One thing is clear: I do not want to see, and we will not have, US-style farming in this country," he said. Yet some argue US-style farms have already taken over British agriculture, largely under the radar and without a national debate.

Gove was reacting to last week’s report by the Bureau of Investigative Journalism which revealed there are now 800 “mega-farms” in the UK, huge industrial units mimicking the feedlots of California or Texas. The biggest can house more than a million chickens, 20,000 pigs or 2,000 cattle. Their emergence is part of a 26 per cent rise in intensive farming in the UK in the last six years.

This rise is mainly due to Britain’s insatiable appetite for chicken. In the 1950s, it was seen as an elite food and was expensive. Just a million were produced a year. Then, intensive farming methods were imported from the US. In 1959, the first fast-processing "poultry factory" was opened in Aldershot. By 1965, the price of poultry had fallen by nearly a third, causing demand to soar. By 1990, almost a quarter of the meat eaten in Britain was chicken or turkey. As birds can be brought to slaughter much more quickly than cows or sheep, it remained cheaper than beef or lamb.

People also began to change their meat-eating habits for health reasons. From the 1970s, government campaigns advised people to eat less fatty red meat. Chicken was seen as a leaner, healthier, alternative.

Now, it is the nation’s favourite meat. Last year, nearly a billion birds were slaughtered and another 400 million imported. Five companies – two of which are owned by multinationals - control most of the poultry production in the UK. Industrial farms are clustered in pockets of the country near their abattoirs and factories. It is causing conflict in the countryside, as local people and campaign groups say they are a blight on the landscape and complain of the smells and disturbance of lorries bringing in grain or taking birds to the abattoir.

Professor Tim Lang of the Centre for Food Policy at City University believes the change to intensive farming has entrenched cheap chicken into our culture. "The more cheap meat these farms produce, the more people eat, the more cheap meat becomes part of the culture and lifestyle. We now have chicken and chips, chicken nuggets, chicken burgers. Chicken is the processed meat of choice," he says. Free range chicken accounts for 3 per cent of the market. Organic – which has the highest animal welfare standards – makes up just 1 per cent.

Yet the actual meat has changed since intensive farms arrived. Experts tested chicken from such farms in 2008 and found it had twice as much fat, a third less protein and a third more calories than in 1940. Gram for gram, it had as much fat as a Big Mac.

Chickens grown for meat are kept in computer-controlled warehouses, with up to 19 birds per square metre (roughly the same amount of space as an A4 piece of paper per bird). They are fed additive-filled, high protein food and the temperature and humidity is controlled so they gain weight. They are taken to be slaughtered when they are five to six weeks old.

Farmers and the food industry say this is the most efficient and green way to produce the meat people want. Inside sheds, the birds are protected from predators while disease and pollution can be controlled. Putting these birds out to pasture would use up more land – land which could be used for houses, parks or kept as countryside. Last June, a Defra survey counted 173 million poultry birds on the ground at that point – though as there are many "crops" of chicken many more are slaughtered in total. If we wanted to raise all those birds to organic conditions, we would take up the same amount of space as the whole of the island of Anglesey.

Animal welfare campaigners say the current "factory farming" system is cruel. Chickens want to feel the sun on their feathers, roll in dust and forage for seeds. Cramped inside a shed, they become stressed and start injuring or cannibalising one other. Food poisoning bugs such as E.coli or campylobacter, many of which are becoming resistant to antibiotics, can spread quickly through a herd. Some 63 per cent of supermarket chickens are now infected with campylobacter, the latest government testing shows, although this has decreased since last year.

The latest report, written by the House of Lords’ EU Energy and Environment Sub-Committee, said polls show 80 per cent or more of the UK public want animal welfare standards to be maintained or improved post-Brexit. Yet many consumers are not aware of the difference between intensive and organic farming – and may not be willing to pay a price for premium welfare products, it said.

Lang believes debate should be opened again. People need to understand where their meat comes from and whether they are comfortable with the methods used to make it. The rise in intensive farming is driven by our choices, with food companies and supermarkets acting as our brokers. “If we don’t like it, we must vote with our purses, demand retailers change their contracts and specifications in our name," he says.

‘With Brexit looming, British consumers need to be very clear: do they want animal welfare standards to rise or get swept away in pursuit of cheaper food?’