Warnings that Calais could prove to be Brexit’s hidden fireball should not come as a great surprise. While Ireland has, for political reasons, long proved to be the focus of anxiety about the UK/EU goods trade after we leave the bloc, far more traffic crosses the Channel, and, in the worst-case scenario, blocked ports at Calais or Dover could strangle Britain’s economy, manufacturing and food supply.

The manager of the port of Calais and the region’s president were not doom-mongering for their own amusement when they privately warned MEPs about the oncoming dangers. Calais’s boss, Jean-Marc Puissesseau, is entirely correct that compulsory tariff checks (as a result of leaving the customs union) and phytosanitary checks (as a result of leaving the single market) will simply be unavoidable legal realities if the government insists on putting us outside key economic instruments. His predictions of 30-mile tailbacks and rotting food – not to mention the businesses that will be crippled by the disruption to just-in-time manufacturing – must be treated with the utmost seriousness.

Although a new customs system for British ports is being planned, it is not close to being ready

Except the government still has its fingers in its ears. Despite her reasonable tone at her Mansion House speech this month, the prime minister has settled in her bunker, refuses to heed warnings, and continues to offer solutions she knows the EU cannot accept. The UK’s only concrete proposals so far – a “customs partnership” and unprecedented technological solutions – have been dismissed outright in Brussels.

If we do not secure a soft Brexit at the end of the transition, we will either have a hard Brexit or no deal at all. A hard Brexit – which is the EU’s expectation if we sign a Canada-style free trade agreement – means compulsory border controls. Outside the customs union, all goods must face rules-of-origin checks at either Dover or Calais to ensure that goods from the rest of the world pay the correct EU tariffs. This will be the case even if UK goods face no tariffs.

Outside the single market, the UK may not implement future EU standards, and could, for example, import currently forbidden US foodstuffs as part of a new trade deal. All agricultural goods crossing the Channel must consequently be inspected in order to guarantee the security and integrity of the EU’s standards and supply chains.

Calais after Brexit 'could be 10 times worse than Irish border' Read more

If we leave the EU without any deal at all – which the government still insists is a possibility – there will be no agreements whatsoever to facilitate trade or give preferential access to UK goods arriving in Calais. We will also not have the luxury of a two-year transition to prepare the new infrastructure.

In the event of either a no-deal or hard Brexit – the latter of which is, of course, the government’s declared choice – the infrastructure at our ports will be placed under unprecedented strain. Even if the government chooses not to implement new controls on incoming goods, the French and Belgian authorities will be compelled to do so, which could hold up lorries on the British side before they even board the ferry.

Facebook Twitter Pinterest ‘Despite her reasonable tone at her Mansion House speech earlier this month, the prime minister has settled in her bunker.’ Photograph: Peter Nicholls/AFP/Getty

Although a new customs system for British ports is being planned, it is not close to being ready, and there is little evidence of the new lorry parks – or thousands of extra customs officials – that will be required the day the transition ends. Puissesseau stressed that goods from the UK would be treated the same as those from anywhere else in the world – causing potential disaster to British exporters who depend on participating in complex supply chains with roll-on, roll-off traffic.

As ever with Brexit, the solution is staring us in the face. The EU has made clear that the only way of keeping the Irish border invisible is to keep Northern Ireland in a “common regulatory area” with the EU – that is, a full customs union with the EU’s common external tariff, and a single market in goods. If that is the only way to keep the Irish border open and seamless, without any checks, the same is also true of Britain’s borders with France and Belgium. Brussels insists that a seamless border requires both a customs union and a single market. No other kind of frictionless border exists anywhere on Earth. The entire UK must stay in both.

Record number of lorries using Dover port raises fears of Brexit delays Read more

May no longer pretends that we will retain the “exact same benefits” of the EU, as David Davis used to. But last week at Mansion House she did promise five tests for Brexit. One was to protect jobs and security; another was to keep Britain outward-looking. A broken border and rotting food at Calais do not qualify. Nor do they remotely meet any democratic mandate. If this is the best the government can think of, it must think again. So, ultimately, might we.

• Jonathan Lis is deputy director of the thinktank British Influence