If Boris Johnson was elected on a single slogan, it was ‘Get Brexit done’. He then claimed it was done at the end of 2019 in the terms for leaving the EU he agreed. Not so. Today legislation will be introduced by the Foreign Secretary Liz Truss to unilaterally overhaul a central pillar of the UK’s negotiated exit from the EU, the Northern Ireland Protocol – which is seen by the EU, whatever the government may claim, as a breach of the UK’s international treaty obligations.

Economic relations with the EU, still the biggest market for our exporters by a country mile, were already bad. They are about to become appallingly bad. As the UK’s former ambassador to the EU Sir Ivan Rogers said in a magisterial lecture on Thursday: ‘The EU… is bound to commence legal proceedings… [It] will view the threat to rewrite the Protocol unilaterally as self-evidently in bad faith, as an extraordinary hostile step to take at this geopolitical juncture and as warranting retaliatory safeguard measures.’

Rogers anticipates the EU commencing legal proceedings against the UK within days, a freezing of all the important talks on further trade and research co-operation between the UK and EU, and selective trade sanctions by the EU against the UK. At a time of looming recession, and when the Ukraine catastrophe would suggest cordiality is the better policy, all this is pretty disastrous.

Johnson argues that he could not have known when he agreed to the Northern Ireland Protocol’s new economic border in the Irish Sea between Great Britain and Northern Ireland that this economic border would actually materialise. At a time when trust in politicians is at a low, this phoney naivety is hardly designed to restore faith.

But even when milk is spilt on purpose, best not to weep. Better to look at how to restore the damage. And the harm is that Northern Ireland’s unionist parties, led by the DUP, are refusing to allow the province’s devolved government to function unless and until the Protocol is ripped up. That in turn threatens to undermine the Belfast Good Friday Agreement, the historic constitutional settlement that ended decades of endemic violence between the Catholic republican and Protestant unionist communities.

Johnson’s position that trade relations with the EU are less important than the risk of a resumption of violence on the island of Ireland is reasonable. The question is whether unilaterally legislating away the Protocol is the optimal response. Maybe Johnson is correct that Brussels has not shown enough imagination and flexibility in negotiations. Maybe there is an element of truth that the Protocol represents the expressed will of the EU to make leaving their club as painful as possible, and therefore it is fair for a British government to attempt to amend a legal contract.

But where Johnson is on shaky ground is that within the Protocol there is explicit provision to suspend it, where there are ‘societal difficulties… liable to persist’ via its Article 16. If the looming collapse of the Northern Ireland Assembly represents ‘societal difficulties’ – which surely it does – then the less aggressive act by Johnson would be to trigger Article 16.

So why has he chosen to go the macho route of making a British law instead? The reason is Johnson wants to go further than triggering Article 16: he wants to abolish all and any role for the European Court of Justice in adjudicating whether the integrity of the European single market is being protected and whether the terms of the Protocol are being honoured. This is all about the religious conviction of his ultra Brexiter wing, the European Research Group, that Brexit should have expelled the European Court of Justice from every inch of the UK. In other words, all those tedious and cancerous battles with the EU over sovereignty that were supposed to be solved by Brexit are still with us.

And they are set to cause the UK potentially serious economic harm when we can least afford it. Is this the fault of the bloomin’ Remainers who opposed leaving the EU? That would be an absurd claim, though I have heard it made by Brexiteers. No, this mess can be laid at the door of those Brexiteers who actually negotiated and agreed to the terms of leaving the EU, and one politician in particular: the Prime Minister.