BREXIT is never done, it seems. While Jacob Rees-Mogg investigates bringing back imperial measurements, Liz Truss has called for a chaotically-fast removal of the hundreds of EU laws that still apply in the UK by the end of next year. But it’s the UK’s refusal to implement fully the Northern Ireland Protocol that remains the most critical and immediate challenge to EU-UK relations.

It was Boris Johnson, back in 2019, who agreed his "oven-ready" deal with Brussels which kept the border between Ireland and Northern Ireland open, and also kept Northern Ireland in the EU’s single market for goods. The remarkable implication of this deal was, and remains, that there would be an internal regulatory and customs border for trade in goods from Britain to Northern Ireland.

The Tories have been chafing at the bit about this internal border ever since, despite it being a consequence of the hard Brexit they chose. Meanwhile, the EU has initiated seven different legal complaints against the UK’s failure to fully implement the Protocol, as the UK has repeatedly and unilaterally extended so-called grace periods for a range of foodstuffs and other goods.

Now, with yet another new Prime Minister in post, the UK and EU are on a deeper collision course. It was Liz Truss herself who introduced the Northern Ireland Protocol Bill to the House of Commons in June this year. The bill would unilaterally override parts of the Protocol, including ending the role of the Court of Justice of the European Union and removing some border checks. Having passed its second reading in July, it is due in the Lords in October, where some peers will doubtless try to amend it.

Brussels has been deeply unimpressed at this renewed threat by the UK to renege on an international agreement – something Mr Johnson did two years ago in the Internal Market Bill, before capitulating and removing the offending clauses. The EU has threatened to “respond with all measures at its disposal” if the bill goes through. This could even mean the suspension of the overarching EU-UK trade deal (though that would take time) and the start of a tit-for-tat trade war between the two sides.

Meanwhile, in Northern Ireland, the new Assembly, elected in May, has been stymied by the Democratic Unionist Party’s refusal to nominate ministers to the Executive until its concerns on the Protocol are resolved. Yet three-quarters of the public in Northern Ireland want a negotiated resolution to the endless Protocol arguments, not a unilateral reneging on an international deal.

There are some in the EU who hope, against their better judgment, that Ms Truss will back off from a major political and economic collision with the EU, not least at a time when so many other big crises top the agenda, notably the interlinked cost-of-living, energy and war in Ukraine ones.

But the choices for Ms Truss are, as so often for Brexit, political and ideological more than anything else. And the key question for her may be the response, if she did a deal with the EU, from her core supporters amongst Tory MPs, especially the Brexit hardliners who backed her. She may calculate she can’t afford to alienate them.

Ms Truss may too, like Mr Johnson before her, hope that a big fight with Brussels will shore up wider public support in the polls, and distract from the other multiplying crises. This would be rash. The economic consequences of a major UK-EU falling out when the economy is already in a parlous state would not be pretty. It’s "no deal" territory all over again. But then this is a Prime Minister bent on cutting taxes, deregulating, increasing debt, and ignoring inequality, so rationality may not win out.

And if Ms Truss really does move rapidly on removing workers’ rights or environmental protections, then that is likely to open another front with the EU, since the EU-UK trade deal includes a commitment to a level playing field on labour, climate and competition issues. Singapore-on-Thames will not cut it in Brussels.

Still, the EU is working hard to keep a space open for possible talks this autumn. Last week, the EU’s Brexit supremo, Commissioner Maroš Šefčovič, said he was looking at ways to make border checks almost invisible. Ms Truss also met with Ireland’s Taoiseach, Micheál Martin, on Sunday before the Queen’s funeral. Ireland is persisting with its deliberately patient diplomacy, repeatedly pointing to the possibility of a negotiated solution, something No Ten claims to want too.

Ms Truss will meet Commission President Ursula von der Leyen today at the UN, having met France’s President Macron yesterday. But will it just be a restating of existing positions? It’s clear that there is a possible deal whereby checks are, to some extent, simplified and made less intrusive. Brussels put forward a set of proposals on this almost a year ago.

But the UK has to decide if it will negotiate in good faith on all these tricky questions of customs bureaucracy, animal and plant health, VAT, medicines and more. If, instead, Ms Truss decides that the Northern Ireland Protocol must be fundamentally re-negotiated, then there is no obvious deal in sight.

This UK-EU stand-off is, of course, being watched closely in Washington. President Biden is likely to make it clear to Ms Truss when they meet at the UN today, as he did to Mr Johnson, that undermining the Protocol risks destabilising the peace process, and damaging UK-US relations.

Ms Truss may show no more inclination than Mr Johnson did to build a serious, strategic UK-EU strategic relationship, preferring instead the ideological unicorn of Britain as a major global player. But weakening the UK’s relations with the US is more alarming. Ms Truss admitted yesterday that a UK-US trade deal is off the cards. But pressure from Mr Biden will still count and could make Ms Truss hesitate before reneging on the deal with the EU. Rational self-interest, global power relations, and Brexit ideology – she will have to choose.

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