The case for any specific safety issues relating to the use of chlorine-based rinses and sprays in chicken production in the US was settled as far back as 2008 when the EU's own food safety body, the European Food Safety Authority (EFSA), concluded "exposure to chlorite residues arising from treated poultry carcasses would be of no safety concern".

It has endured as a means to characterise differences between the US and EU in chicken production processes and pathogen-reduction approaches. The term itself, however, is a misnomer in a number of ways. 

While to chlorinate can mean both to treat with and impregnate with, it is used in this context to describe the use of chlorine as an antimicrobial agent in water to wash and chill chicken carcasses and chicken segments. By contrast, the only rinsing agent UK regulations permit companies to use is potable water. 

Reducing carcass temperature as quickly as possible is critical in poultry processing, and in the US immersion water chilling is the norm. While the water sprays used to wash carcasses in the UK contribute to reducing temperature, air chilling accounts for most of the cooling process in the majority of UK chicken production. 

Extensive use of the term also suggests chlorine is widely used but, according to data published by the US National Chicken Council (NCC), this is not the case. The NCC states chlorine is used in rinses and sprays in less than 5% of processing plants in the US. 

Its usage has also been steadily declining, with other agents such as peracetic acid (PAA), cetylpyridinium chloride (CPC), acidified sodium chlorite (ASC), organic acid rinses and bromine being used. Use of PAA has become particularly widespread, and is identified as the industry standard by Dr Jonathan James, corporate quality assurance manager for Sanderson Farms.

Brexit and the US trade negotiations have resulted in the formation of an unlikely alliance between the UK poultry industry and environmental and animal welfare campaign groups. The argument goes that the use of antimicrobials, including chlorine, is representative of lower food, farming and animal welfare standards in the US and opening the market to American agriculture will undermine higher British standards.

Unsurprisingly, Jim Sumner, chief executive of the US Poultry and Egg Export Council (USAPEEC), takes issue, suggesting contentions made in association with the term chlorinated chicken are a "misrepresentation of our industry and the products we produce".

"The UK has probably been unfairly influenced by the EU Commission in its rather politicised attempt to block poultry imports through the years," Sumner adds.

Comparative figures between the US and the UK on the instances of illness from the primary pathogens associated with poultry, particularly salmonella and campylobacter, have been cited by campaigners and challenged by political proponents of lifting the ban on agricultural imports from the US. 

The reason behind this, according to Dr Edward Fox, senior lecturer in the Department of Applied Sciences of Northumbria University, is making definitive comparisons regarding illnesses that can have multiple causes and be measured in so many different ways is extremely complicated. 

"The problem and why there are a lot of different numbers thrown about is it's a really complex number to elucidate out," Dr Fox says. As these illnesses are often self-limiting and are therefore unreported, as well as being subject to many other variables, "the actual number of cases of people who get sick with these diseases every year is not 100% clear," he continues. 

"We know how many people present at hospitals and these figures are used in really complex disease estimations that include a number of other different factors to estimate the number of infections that these organisms cause. And when you try to compare different countries, this is complicated."

A "dual tariff" approach on imported foods, imposing a higher tariff on those that do not meet higher food production and animal welfare standards, has been put forward as a solution by certain UK government ministers concerned about the impact on domestic farming of a US free trade deal including agriculture. This would include hormone-treated beef and "chlorinated" or antimicrobial-rinsed chicken.

Sumner expects the dual tariff idea to be given short shrift by US negotiators. "I don't know whether governments are capable of establishing tariff levels on such subjective issues as animal welfare. What we have said all along is label the product for what it is and let the consumers be the judge."

What this proposal clearly represents is that, as far as the UK government is concerned, including Defra Secretary George Eustice, food safety is not the issue.

The former chief scientific adviser to Defra, Sir Ian Boyd, not only appears to concur with Eustice but also with Sumner, telling Sky News on his departure last August "from a health perspective, there really isn't a problem with chlorinated chicken. The issue is about production processes and animal welfare, and that is a values-based choice that people need to make."

Some campaigners dismissed these remarks – which Defra stressed Boyd made in a personal capacity – as politically motivated, and have since doubled down on food safety fears. UK farming trade body the National Farmers' Union launched an online petition, recently topping 1m signatures, urging the Government to ensure future trade deals do not lead to an increase in food imports that would be illegal under current regulations.

As all these regulations are currently defined by EU law, the petition and the discussion of chlorinated chicken are a continuation of the long-running Brexit arguments. "It's hogwash and it's politics," says Mike Cockrell, chief financial officer at US chicken producer Sanderson Farms. "I would put our safety record up against anyone's. We constantly test for salmonella, campylobacter and other bugs and our plants perform very well. And I'm willing to bet, even though I've not done the research, they compare very well to those based in Europe."

The NFU does not use the term chlorinated chicken on the petition webpage, but has used it in other contexts. Is this simply a shorthand way for campaigners to encapsulate what they see as critical differences in agricultural methods? Or the use of a term that does not describe what is actually taking place all that accurately and may convey additional menace to many consumers? That is also a matter of opinion.

Richard Griffiths, chief executive of UK trade body the British Poultry Council (BPC), appears to have more common ground with his US counterparts regarding the food safety question than with campaigners. "We've said it a number of times around the US debate, the issue is not safety," Griffiths says.

Griffiths also concedes the use of the term chlorinated chicken is not helpful and says he avoids using it as shorthand for lower standards. "I like to make a distinction because chlorinated chicken has become a proxy for 'things that are bad about Brexit' so it applies to so much more than chicken. So, I try to make a distinction between that general use and the specific use of chlorine."

However, Griffiths' approach has been to broaden the issue further still, beyond agricultural and animal welfare standards to societal challenges regarding food poverty and nutrition, and to government responsibility, not only to ensure its people are fed, but that everyone has access to "quality food". 

While UK attitudes to animal welfare, sometimes regarded in other countries as over-sentimentalised, are significant in the debate about a US deal on agriculture, Griffiths says policymakers need "to put people at the heart of our food system".

This helps cement the current love-in between the UK poultry industry and local campaigners, who were railing against the sector a few years ago when Asda succeeded in marketing its infamous GBP2 chicken. 

Griffiths concedes the UK poultry industry represents a leading example of "efficient, intensive production" in UK agriculture, and freely admits when the common cause is no longer there, the relationship with campaigners and pressure groups will revert to something rather more challenging.

The truce and shared objectives are likely to remain while there is hope the UK will be prepared to exclude agricultural products from a US trade deal. Despite the UK government recently conceding to the NFU's demand to establish a Trade, Food and Farming Standards Commission to advise on how to maintain high food production standards in any future trade deals, the US poultry sector clearly has greater confidence its trade negotiators will hold firm. 

Meanwhile, the UK government is considering the issue of chlorinated chicken in the broadest context of all, and faith among British farmers that Boris Johnson has their back is in rather short supply.