In Northern Ireland, 56% of those who took part in the referendum on membership of the European Union voted to remain. For the majority, the freedom for people and goods to come and go without checks across the Irish border carries the momentous freight of national identity; it goes to the heart of the peace settlement. The UK government knows this – which is why the prime minister has promised a contradiction, that what will become the border with the EU will remain frictionless, despite also promising, to please Brexiteers, that Great Britain and Northern Ireland will be outside the customs union and single market.

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The idea that you can have a frictionless, open border without customs arrangements that match exactly (a union) or near as damn it (regulatory alignment) on either side is a myth. But while the politics of the border have been extensively discussed, the practical importance of the customs union is still not widely understood. While it may sound technical, what it controls is as basic as bread and milk. We discuss it in the abstract. We need to talk about the effect on people.

The Northern Irish economy depends on its agri-food exports. The lion’s share of the £1.15bn a year in Northern Irish exports to the EU flows across the Irish border, moving through one of the 275 or so crossing points along its 300-mile stretch. Small tracks, minor roads and EU-funded dual carriageways are used for the thousands of daily movements of people, lorries and animals back and forth. Forty percent of lamb reared in the north travels over to the south for processing, as does up to a third of Northern Ireland’s milk production. Food and drink is the north’s largest manufacturing sector and one in 10 jobs depend on it.

Cutting off the flow of food into Northern Ireland, meanwhile, is unprecedented outside of the circumstances of war. Just three supermarket chains feed Northern Ireland, to all intents and purposes – Tesco, Asda and Sainsbury’s supply 70% of the country’s grocery sales, providing an uninterrupted stream of fresh food sourced from across continental Europe through Britain. Think on that with the day-to-day impact on families in mind. Even a small increase in delays for inspection would put a spanner in works that have been finely tuned for speed, and could risk the prospect of food rotting before it was processed.

Play Video 0:35 'Like the congestion charge': Boris Johnson on Irish border checks post-Brexit – video

In a report this week, experts from the Chartered Institute of Environmental Health and City University of London dismiss the idea of a technological answer to the customs union problem, aired most recently by foreign secretary Boris Johnson in a radio interview, as a fantasy. At the moment food moves freely through infrastructure that is largely invisible. Even if the political consequences of a new, hard border were surmountable, the practical ones seem unmanageable.

A complex web of port operators, clearance agents and logistics companies currently makes customs declarations for shipments in and out of the EU by filling in online declaration forms for Revenue & Customs. The department was developing a new IT system before the Brexit vote. It needs to. Its creaking 30-year-old system has typically dealt with about 55m customs declarations a year; the new one was only designed to manage up to 150m. Now HMRC estimates that Brexit will create 255m a year. Like nearly all large government IT projects, it has run into problems.

Added to that, Revenue & Customs is in the throes of a major restructuring that is reducing its national network of offices to just 13 regional hubs, with large numbers of staff lost as a consequence. In some parts of the country, if a border force official wants to call on customs officers to go to a port to check a load, they will now be several hours’ drive away. How can that keep essentials flowing?

And even if there were to be some kind of electronic border, could it be properly policed? Checking that goods and their origins are what they say depends not on technology, but on people opening lorries and looking inside, at least some of the time. The control of food and livestock for safety and disease depends on those people being trained specialists. The Irish border has long been a hub for smuggling and EU quota fiddling. The Chartered Institute’s director for Northern Ireland, Gary McFarlane, fears that without proper checks there will be a surge in fraud and crime.

The idea that border checks could be moved to somewhere in the Irish Sea brings its own set of problems. Even if this were politically acceptable – which it is not, especially to the Democratic Unionist party – there is no physical infrastructure to deal with the scale of trade. More than 680,000 tonnes of food passes each way each year between Northern Ireland and mainland Britain, while the majority of Irish exports to the EU from its all-important agri-food sector also go to or pass through Britain. About 40% of Irish farm exports go to the UK. No British plans to build any such infrastructure or recruit enough employees to run it are yet evident.

The head of Dublin port is preparing to build enough new capacity for a fivefold increase in freight that will need to pass through border controls because of Brexit in 2019. That’s how seriously they are taking it. By contrast the UK government behaves in laggardly fashion. Given the likely real-life effects on millions of British people, isn’t this truly baffling? • Felicity Lawrence is a special correspondent for the Guardian