Most people pay little attention to the chain of production that brings food to their plates. Americans eat tonnes of chicken every year, unaware of, or unconcerned by, the chemical rinse applied to its pre-cooked carcass. British consumers might also eat chlorine-washed chicken if they had acquired the habit, but since they haven’t the idea is unappetising. This is a problem in transatlantic trade talks because US agribusiness wants access to UK dining tables. The politics of serving food prepared to US safety standards is tricky on a number of levels. Alongside chlorinated chicken, the use of growth-promoting hormones and antibiotics in meat production are more tightly regulated in Europe than in many countries that want to export their surplus meat. After Brexit, the UK can abandon EU standards, but if it does so it will find its produce barred from continental markets.

Eurosceptics call that protectionism, and partly it is. European food safety standards have a scientific rationale – the idea is that hygiene should be maintained all along the chain, not dealt with at the last stage by blasting microbes with disinfectant. But those rules are bundled up with a system that insulates the sector from global competition. The EU subsidises farmers, recognising how destabilising it would be if a glut of cheap American produce was dumped on their markets. Some Eurosceptics embrace that destabilisation as an economic tonic, others deny it would happen. Pro-leave campaigners advertised cheaper food as a benefit of Brexit, omitting to explain that a price would be paid by farmers. That sleight of hand was easier to accomplish in propaganda than as government policy. The National Farmers’ Union vehemently opposes lowering regulatory barriers to American produce. The NFU president, Minette Batters, this week said doing so would be “insane” and “morally bankrupt”. Downing Street insists high standards will be maintained, but Boris Johnson wants a trade deal with the US and is not renowned for keeping his word. George Eustice, the environment secretary, tries to assuage farmers’ fears without giving them explicit guarantees.

The EU has been less ambiguous. Michel Barnier insists that the application of common standards across the single market will not be compromised as a favour to Britain. Access depends on alignment and, as Mr Barnier noted this week, the geographical proximity of the UK to the rest of Europe makes the enforcement of standards across post-Brexit borders all the more important. As the European commission sees it, such a near neighbour could easily become an entrepôt for substandard produce. That explains EU frustration at Tory MPs’ habit of downplaying or dismissing the requirement for controls at Irish Sea ports. The preservation of an all-Ireland regulatory space under the withdrawal agreement makes such checks necessary once mainland Britain diverges from EU rules. Northern Ireland’s Unionists hate that idea and Mr Johnson hates admitting that he let them down, but those are not grounds to renege on a treaty.

The prime minister can try persuading Mr Barnier to turn a blind eye to changes in UK food standards; he can try persuading British consumers to eat chlorinated chicken; he can try persuading farmers to accept being undercut by American imports. But he will struggle in two ways. First, standards are settled by law, not trust. Second, Mr Johnson has proved that he cannot in any case be trusted. The prime minister keeps serving up rehashed Brexit promises, but they get ever harder to swallow.