Chlorinated chicken has become totemic once again in talks between the UK and the EU about their post-Brexit relationship, thanks to the EU inserting a new clause into its negotiating mandate. The clause will require the UK to maintain a ban on poultry treated with disinfectant if it wants a trade deal with its nearest neighbours.

The US allows its industrial meat producers to wash their birds in chlorine or other disinfecting acid solutions after slaughter to kill the food-poisoning bugs they often carry, having been contaminated by chicken faeces during processing. Persuading British negotiators to drop the ban and accept US food standards is a high priority for the US in a trade deal with the UK.

The battle is about much more than chlorinated, or more accurately chlorine-washed chicken. This particular US agricultural export keeps rearing its head because it encapsulates much wider arguments about the future of trade between the different power blocs.

Since most British consumers associate chlorine with the bleach they pour down their lavatories, the Americans know they have lost the PR war on that particular chemical treatment, and are now promoting the line that their farmers don’t use chlorine much any more, but favour other disinfectants such as lactic acid and peracetic acid.

The US is the world’s largest producer of poultry meat, and nearly one fifth of what it produces is exported. Its lower welfare standards enable it to achieve some of the cheapest production in the world.

For Americans one of the chief prizes of Brexit is the opportunity to break open a major European economy to its huge surplus in agricultural goods. It has experienced years of failed trade talks with Europe in which the EU has refused to weaken its food and farming standards to bring them in line with US ones, whether on genetic modification or gene-editing, growth-promoting hormones in beef and pig production, a series of pesticides, antibiotics and other veterinary drugs, and on labelling.

The EU takes a fundamentally different view on food safety to the US. It applies the precautionary principle, banning substances and processes that are potentially harmful to humans and nature until they are proved safe. The US tends to approve chemicals until they are proven harmful and allows its producers to clean up at the end of the chain. It sees the EU objections to its goods as fundamentally protectionist.

Conservative Brexiters see European objections to US standards in the same light. They have argued that once free from burdensome EU regulations, British consumers will benefit from much cheaper food. For them the biggest prize in return is greater access to US markets for UK financial services.

The British poultry industry is worth £7.2bn a year. Like the US industry, it suffers from persistent problems with contamination. (The US has more E coli and salmonella, the UK more campylobacter; in both countries poultry is one of the main cause of food poisoning.)

British farmers know that if US imports of cheap meat produced to different hygiene and welfare standards are allowed, they will be fatally undercut. The European industry fears the same.

For Boris Johnson, who happily played the cuddly animal card with his new dog, Dilyn, during the election, ignoring British consumer attitudes to welfare will be difficult, especially with the EU making clear the UK cannot have it both ways.