Is Northern Ireland the first crack in the dam? There is no solution to hard Brexit along the Irish border. Negotiators have been chasing this will-o-the-wisp for over a year. They have not found it because it does not exist. A border is a border, it is not “not-a-border”. It means barriers, checks, queues, papers, regulations, tariffs. No one wants it in practice. Does anyone want it in theory?

The trouble is politics. If Theresa May agrees special status for Northern Ireland to remain in a trading union with Ireland it will effectively “move the border” to Belfast. Her fragile Unionist coalition collapses. If the Irish prime minister, Leo Varadkar, fails to win such special status and sees a border installed, his confidence and supply government collapses. There is no way round this. It is the Schleswig-Holstein question of the age.

There can be no iron curtain across the Irish countryside. Not 10% of the British public would want that. Even the fiendishly complex use of electronic tags would still leave in place the fact that leaving a customs union would mean monitoring different tariffs and regulations north and south of the border. It would be a licence to smuggling and piracy.

Much of Ireland’s trade passes through Northern Ireland, to the UK and on to Europe. Any sort of border – physical or regulatory – would mean massive distortion. British firms would decamp to Dublin to get inside the EU. Foreign and Irish firms would decamp to Belfast to get to the outside world. Any compromise such as special “free port” status for Northern Ireland, which would give it a foot in both camps, would be furiously opposed by governments across the EU. They could not tolerate a corner of the EU free of harmonised standards or with separate external tariffs.

So what is supposed to happen? The answer is easy. It is for the British government to announce it will remain in a customs union with Dublin. Since Dublin’s Varadkar means to stay in the EU, that means no trading barrier between Northern Ireland and the EU. But since May must retain Unionist support, she cannot admit any trading barrier between Northern Ireland and Great Britain. Ergo, there can then be no trading barrier between the UK and the EU.

Britain has to remain in the customs union with the rest of Europe. That is what should be on the Brussels negotiating agenda next month. And that is two-thirds of the way to remaining in the single market. That is what polls show the overwhelming majority of parliament and British public opinion, including those who voted Brexit, actually want.

Where is democracy?

• Simon Jenkins is a Guardian columnist