Boris Johnson was not honest about the Irish border. What is he going to do about it?

Apologies make cynical history, but Boris Johnson has a big one to make, and fast. He must apologise to Northern Ireland’s unionists that he did not mean it last year when he pledged “no border” down the Irish Sea. As the Good Friday agreement negotiator, Jonathan Powell, wrote on Sunday, this was a lie. Johnson had just told the Irish government that the Good Friday deal held and there would be no border on the island of Ireland. Given Britain’s intention to leave the EU’s customs union, the two statements were incompatible, and Johnson knew it. Every truck on the Belfast ferry knows it, too.

The current Belfast riots have invoked the usual platitudes. The Irish taoiseach, Micheál Martin, has called for calm. Joe Biden has offered concern. Everyone is outraged that children are being encouraged to attack the police. Even Prince Philip’s death has been cited as a call for restraint. Deprivation, local political grievances, poor relationships with the police – these are all factors behind the disturbances. But every act of violence also carries the same word: exasperation. Will someone answer the question? Johnson lied, and what is Britain going to do about it?

The writing was on the wall the moment Johnson decided his Tory leadership bid would be enhanced with a bit of macho xenophobia. He would not just leave the EU but also renounce the entire European economic zone – customs union, single market and all. This would, of necessity, put Northern Ireland in a singular position and encourage a closer economic relationship with the south. Otherwise the “peace walls” of Belfast’s Falls and Shankhill Roads would have to stretch in a Trump-like trade barrier across the fields of Down and Armagh.

Related: Peace in Northern Ireland is in danger – Johnson’s lies and inaction offer no help | Jonathan Powell

The Good Friday agreement was a bid towards a lasting Irish rapprochement. Johnson’s Northern Ireland protocol went further. Following the logic of his version of Brexit, it accepted the necessity of the north’s increased reunion with the south. Even with last year’s no-tariff deal with the EU, trade compatibility would require a customs barrier down the Irish Sea.

As a policy towards a future Ireland this was sound. But it meant a moment of painful truth for unionists. They would have to admit that their economic future might lie with Dublin, not with London. Johnson lacked the guts to tell them this. It was the loyalist rioters who got the message.

There is only one answer to the riots, and it is not platitude. The prime minister must carry his Brexit policy to its logical conclusion. Northern Ireland must abandon its cosily antique relationship with Great Britain, still stuck in 1922, and build a lasting accord with the south. Such an eventuality would be a true plus to Brexit.

There is, of course, an alternative. Johnson could apologise for a different mistake. He could admit that leaving the European single market was a crass error, and swiftly renegotiate trade compatibility across the board with the EU. Borders could then remain open. The unionist tail would thus wag the British dog back into a more sensible Brexit. There is no point in waffling about peace. The question is which mistake will Johnson correct.

• Simon Jenkins is a Guardian columnist