THE most visible evidence of the European Union’s single market is its dismantling of border controls. Norway is in the single market, except for some agriculture and fish. And unlike Britain and Ireland, it is in the Schengen passport-free travel area. Yet as it is not in the customs union, Norway has border controls and its exports must be checked to comply with EU rules of origin.

In December Theresa May promised to have no physical infrastructure or checks and controls on the border between Northern Ireland and the Irish Republic. Yet she wants to leave the single market and customs union. Brexiteers claim the Irish border can be kept invisible by smart technology, number-plate recognition and a system of authorised economic operators and trusted traders.

Such ideas draw hollow laughs from seasoned customs officers at Svinesund, on the border between Norway and Sweden. A high degree of sophistication and trust makes this one of the world’s most porous frontiers. Most cars whizz through unchecked, with many heading for the region’s largest supermarket in search of alcohol, cigarettes and sugary drinks, which are taxed more heavily in Norway than Sweden. In theory there is a limit to what they are allowed to bring back, but in practice it is tricky to enforce.

When it comes to lorries, though, controls are firmly in place. Norway’s customs service, known as Toll, has expanded in the past two decades. The average lorry takes around four minutes to clear customs and few are pulled in for full inspection. But every one stops at Svinesund, which handles over 70% of all commercial traffic across this border. The mostly eastern European drivers, few of whom speak English, let alone Norwegian, then queue to have their papers checked. This summer a trial of an automated electronic system will begin in Orje, north of Svinesund, but even here most lorries will stop.

Kristen Hoiberget, the customs boss at Svinesund, says his officers rely on intelligence and instinct in deciding when to check vehicles. Svinesund usually has 100 confiscated cars in its car park; Mr Hoiberget says he destroys 500,000 litres of smuggled alcohol each year. He reckons it takes two years to train recruits, and ten before they know how best to spot drug-smugglers or frauds involving VAT refunds. Yet Britain is due to leave the EU in 13 months’ time. And only 40 or so roads cross the Norway-Sweden border, compared with some 275 crossings in Northern Ireland. No wonder many think the promise to avoid any Irish border must mean a softer Brexit.