The disturbing prospect of chlorine-washed chickens from the US going on sale in British shops in a post-Brexit trade deal last week sparked an explosive row at the heart of Government.

But beyond the politics lies the story of why American poultry needs such drastic chemical treatment – and of the horrendous conditions at the farms where they are bred and reared.

Packed in: Chickens at the North Carolina farm run by Craig Watts. He says their flesh would rot due to the conditions

Now whistleblower farmers have revealed the full horror of the suffering to The Mail on Sunday, including how:

Tens of thousands of super-sized 'Frankenstein' birds are crammed in vast warehouses.

The chickens, which weigh up to 9lb, often buckle under their weight and must live without natural sunlight.

Chickens frequently die before they reach maturity and many are left covered in their own faeces, turning warehouses into vile breeding grounds for disease.

Unlike in the UK and Europe, there are no minimum space requirements for breeding chickens in the US. America also does not have any rules governing lighting levels in the sheds and, crucially, its farms have no maximum allowed level of ammonia, which indicates how much urine and faecal matter is present. This means there is no limit on how much can fester inside the sheds.

There is no legal requirement to wash US chickens in chlorine or other disinfectants, but 97 per cent of its birds are cleaned in this way after slaughter.

The possibility of US chickens being sold in Britain after a post-Brexit trade deal sparked a huge Cabinet row, with Environment Secretary Michael Gove speaking out against the move while Trade Secretary Liam Fox insisted the chlorine-rinsed meat was safe.

Dr Fox, who met leading politicians and businessmen in the US last week, sparked fury from the public and other Ministers after he signalled he would be in favour of dropping the EU's ban on importing chicken from the US if it proved to be a barrier to securing a post-Brexit trade deal with the US.

The furore exposed just how difficult it will be for Britain to quickly strike new deals with foreign powers once we leave the EU in 2019. Now whistleblowers have offered a disturbing insight into the £70 billion US poultry industry, which is controlled by the big agricultural firms lobbying to sell their meat to Britain.

North Carolina farmer Craig Watts, 51, told the MoS: 'The birds are too heavy to stand because they have been bred for breast meat and nothing else so they spend their lives squatting. It's like two toothpicks sticking out of a grape.

This image shows the chickens being doused in chlorine at a farm in the United States

'They spend 95 per cent of their time sitting on the litter, a mixture of pine shavings and faecal matter from that flock and prior flocks.'

He said 1,000 of the 30,000 chickens he raised every six weeks would die before they reached maturity. Many become infected on the underside of their chest because of contact with the litter.

He added: 'Their flesh would rot and, when you have them crammed in so tight, they will walk over other birds if they want to get to the food or scratch the others and cause a wound. It is awful.'

Mr Watts, a married father- of-three, quit the poultry business two years ago because he was disgusted by its practices.

Nearly all America's chicken farmers are under contract with big producers who supply them with chicks, feed and equipment. The firms dictate what the farmers can do and are paid according to a 'tournament system' that pits farmers against each other. The farmer who produces the most meat with the least feed comes top. A less efficient farmer will have money deducted from his base pay.

Critics say this system fosters unhygienic practices because it forces poultry farmers to cut corners on animal welfare to maximise their income per flock. Campaigners who work with farmers to improve standards say many are too scared to speak out for fear of having their contracts terminated.

One farmer, John – not his real name – is currently part of a class action lawsuit by chicken farmers against big US poultry companies.

He said farmers earn between £9,000 and £30,000 a year even though they work 16-hour days.

The chickens, which weigh up to 9lb, often buckle under their weight and must live without natural sunlight

This is a whole cooked canned chicken that is sold in US stores

'Last year [chicken firm] Pilgrim's Pride paid $2.2 billion (£1.7 billion) in dividends and we're going on 20 years since we've had an increase in base pay. We get around five cents [4p] a pound of meat. Seventy per cent of poultry growers live below the poverty level.'

Campaigners who have infiltrated chicken farms in the past two years have secretly filmed shocking abuse and cruel conditions at farms across America.

Footage shot at a farm in Georgia last month by the Humane Society of the United States showed the owner of the farm bludgeoning chickens with a metal rod.

The chicken shed at the farm also appeared to be badly overcrowded. Many birds seemed to be suffering from severe leg problems and some were unable to walk to reach food and water. Yet in the US – as in Britain – consumer demand for chicken continues to grow. Nine billion chickens were slaughtered in the US last year.

US chickens have more than tripled in size since 1957, according to academic studies. The birds cost 20 per cent less than British chickens, which are typically one third smaller than US birds. Major poultry producers have cross-bred and interbred birds in recent decades to create 'mutant' chickens which grow larger in a shorter space of time and need less feed.

Peter Stevenson, chief policy adviser for British-based welfare organisation Compassion in World Farming, said: 'This genetic engineering has created unnatural chickens which give you larger breasts or bigger cuts of meat.

'These chickens grow far too large too quickly and they cannot move around. They end up sitting in their own waste.'

The animals bring more faecal matter to the slaughterhouse with them, posing the food safety risk which requires the birds to be washed in chlorine or similar chemicals. However, the US poultry industry maintains that its birds are healthier than ever.

Jim Sumner, president of the US Poultry & Egg Export Council, said US chickens are treated humanely and are only washed with chlorine to make them 'extra safe' for consumers because it guards against food poisoning bugs. He said the process was not harmful to consumers, adding: 'Sometimes these [animal welfare] organisations do not have a thorough understanding of the process or scientific facts.'

Another reason poultry in the US is chlorinated is that farmers are not required to vaccinate against diseases such as Salmonella. Britain and the EU have widespread vaccination programmes.

This image shows the chickens on the way to being slaughtered by electrocution

Supporters of chlorine-washed chicken point out both the US and European food safety authorities have declared the chemicals used to wash chickens in the States do not pose any risk to human health. Tom Super, spokesman for the US National Chicken Council, said: 'Its [chlorine's] use makes the chicken safer. It is not present in the final product, it poses zero health risk.'

But Jaydee Hanson, a senior policy analyst at the US Center for Food Safety, said: 'These chemicals are basically like the ones we put in our toilets to clean them. The question is why are chickens so contaminated in the first place. And the issue is that we are not doing a good job of raising chickens.'

Leah Garces, of the Global Animal Partnership, an animal welfare group, added: 'The fact we have to wash our food in chlorine to make it safe indicates that we are not doing farming right in the first place. It indicates how unhealthy we are raising our birds.'

While UK chicken farmers are not wholly free from criticism from animal welfare campaigners, there are strict regulations that must be followed. In the UK and Europe, poultry farmers must not keep more than 17 chickens per square metre in their sheds. There are also rules governing available natural light, temperature and the maximum levels of ammonia.

In the US there is not one single piece of federal law that governs how to raise chickens. There is not even a law which states that chickens must be stunned unconscious before they are slaughtered, although it is common practice.

There are concerns that if American chicken is allowed into the country, British farmers will be forced to dilute their welfare standards to compete with the cheaper meat.

Shraddha Kaul, of the British Poultry Council, said: 'We strongly reject any move to import chlorine-washed chickens as part of a makeweight in trade negotiations with the US.

'Chlorine is used as a catch-all. It is an approach which means it doesn't matter how badly you treat your chicken, you can just clean it away at the end of the process.'