An Australorp chicken setting eggs in the hen house of Chef Eric Skokan owner of two restaurants in Boulder, the Black Cat and Bramble & Hare, that are farm to table from the families Certified Organic Black Cat Farm in Longmont. May 8, 2017, Longmont, Colorado. Joe Amon | Denver Post | Getty Images

The prospect of a post-Brexit trade deal between the U.S. and U.K. could ultimately depend on the outcome of a controversial debate about whether it is acceptable to wash chicken in chlorine. In setting out its "negotiating objectives" earlier this year, the U.S. suggested Britain should not expect to enjoy softer treatment than any other ally when it comes to chlorinated chicken exported from America. The U.K. government has repeatedly denied it would accept lower food standards post-Brexit, but senior lawmakers in the ruling Conservative Party appear to be split on the issue. International Trade Secretary Liam Fox has hinted the U.K. could accept chlorine-washed chicken in a post-Brexit trade deal, while Environment Secretary Michael Gove — one of the leading candidates to become prime minister — has insisted it would be a "red line" in future trade talks with Washington. So, what is chlorinated chicken? Is it safe to eat? And why does it matter to a potential U.S.-U.K. trade deal? CNBC takes a look at all you need to know.

What is chlorinated chicken?

Chlorinated chicken, or chlorine-washed chicken, refers to poultry that has been treated with antimicrobial rinses in order to remove harmful bacteria. These rinses — often referred to as Pathogen Reduction Treatments (PRTs) in the industry — are designed to kill potentially harmful organisms such as E. coli, salmonella and campylobacter on the surface of the chicken. Washing chicken in chlorine has been banned in the European Union (EU) since 1997 amid food safety concerns. The ban has effectively prevented all imports of U.S. chicken meat generally treated by this process. Instead, the EU insists meat must not be washed with any substance other than water unless it has explicitly been approved by the European Commission.

The National Farmers Union (NFU) believes there are "justifiable fears," both inside and outside the farming community that, after leaving the European Union, Britain could allow the import of food produced to lower standards. John Taggart | Bloomberg | Getty Images

Advocates of the EU's so-called "farm to fork" or "plough to plate" approach argue that it leads to higher standards of hygiene and animal welfare. That's because farmers must take care at every stage of the process rather than relying on a chemical bath to destroy harmful bacteria after chickens are slaughtered. "We've got concerns that if you focus on the end process, you're more likely to be lax when it comes to observing any risks prior to that," Gail Soutar, chief EU exit and international trade adviser of Britain's National Farmers Union (NFU), told CNBC in a telephone interview.

Is the practice safe?

U.S. regulators say yes, chlorine-washed chicken is safe to eat. In fact, the U.S. Department of Agriculture (USDA) has approved several antimicrobial rinses in poultry processing, including chlorine dioxide, acidified sodium chlorite, trisodium phosphate and peroxyacids. European regulators even agree with their U.S. counterparts. The European Food Safety Authority has concluded chlorinated chicken does not pose a health risk to consumers. But, the NFU believes there are "justifiable fears," both inside and outside the farming community that, after leaving the European Union, Britain could allow the import of food produced to lower standards.

We don't need to fall back on this end kind of process treatment… There is absolutely no need for compromise. Sue Davies Strategic policy partner at Which?

"It is entirely legitimate to require high standards of food safety, environmental protection and animal welfare but it must be recognized that these things come at a cost. A cost which is often not borne by overseas competitors, potentially putting U.K. farmers at an unfair disadvantage," the NFU said.

Why does it matter?

A future transatlantic trade agreement is seen as vitally important to the U.K.'s economic future outside the EU. Chlorine-washed chickens are symbolic of wider concerns about food safety, animal welfare, and environmental standards that could become a critical negotiating tool in any post-Brexit deal. Agricultural issues were among the main barriers that prevented an agreement on the Transatlantic Trade and Investment Partnership (TTIP) between the U.S. and the EU in 2016. The U.S. has since cited the ban on the use of PRTs as one of the stumbling blocks to increased trade with the EU. Earlier this year, U.S. ambassador to the U.K. Woody Johnson said fears over chlorine-washed chicken and other stateside farming practices were "inflammatory and misleading."

Packs of "Brexit Selection Freshly Chlorinated Chicken" sit on display at the 'Costupper' Brexit Minimart pop-up store, set up by the People's Vote campaign group, in London, U.K., on Friday, Nov. 23, 2018. Simon Dawson | Bloomberg | Getty Images

In an article published by British newspaper The Telegraph in early March, Johnson urged the U.K. to accept U.S. farming methods after the U.S. published its objectives for a future U.S.-U.K. trade deal. In Europe, it is thought that if the U.K. were to accept treating meat with chlorine as the price of a free trade deal with the U.S., it could make it difficult to sell British meat to the EU. When asked if the prospect of chlorine-washed chicken being introduced into the U.K. post-Brexit raised concerns, Sue Davies, strategic policy partner at consumer group Which?, replied: "Definitely." "We need to be very careful, particularly when it comes to food safety. We have come a long way from the dark days of salmonella scares and BSE (Bovine Spongiform Encephalopathy)." "We don't need to fall back on this end kind of process treatment… There is absolutely no need for compromise," Davies said.

What happens next?