The possible triggering of Article 16 of the Northern Ireland Protocol is now dominating the relationship between the EU and UK, and Anglo-Irish relations.
The main issues are the timing, the motivation, and the EU's response.
It has been a week of fevered briefing by both sides, and it remains impossible to say for certain if the UK will push the button, or if the technical talks between both sides can salvage a deal.
On Sunday, Simon Coveney had said the EU could terminate its free trade agreement with the UK because it derived from the Brexit withdrawal treaty, which a UK trigger would undermine.
"One is contingent on the other," Mr Coveney told RTÉ’s This Week programme. "So that if one is being set aside, there is a danger that the other will also be set aside by the EU."
EU diplomats have, indeed, said that London may be miscalculating if it believed Article 16 would prompt a slow and legalistic response during which nothing much would change.
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At the same time, Taoiseach Micheál Martin warned that speculation on the EU’s response could lead to "self-fulfilling prophecies".
An official in Brussels added: "The EU doesn't want to signal the end of the road for the talks just yet."
Those talks, on medicines, customs formalities, agri-food checks and an enhanced role for Stormont, continued with video conferences between EU and UK officials on Wednesday and Thursday, and another high level meeting in London on Friday between the EU’s chief negotiator Maroš Šefcovic and his UK opposite number Lord David Frost.
Ahead of the meeting Šefcovic briefed EU ambassadors on the state of play. Although he did not spell out how the EU would respond, he echoed Simon Coveney’s remarks that the divorce treaty was only concluded on the basis of sequencing.
In other words, the Irish border (as well as citizens’ rights and the financial settlement) had to be resolved first before both the divorce and the future relationship treaties were finalised.
"That sequencing could be endangered depending on how Article 16 is triggered," says one source present. "So, while [Šefcovic] didn't explicitly promise to say or do anything, he did raise that link."
The European Commission will circulate a paper to national capitals next week setting out the options available if the UK triggers Article 16.
The deepening pessimism in the early part of the week was partly fueled by slow progress in the technical talks.
Šefcovic's team has sought to focus on the EU’s proposals published on 13 October, while the UK side continues to press for the European Court of Justice (ECJ), VAT and competition rules to be part of discussions.
Šefcovic expressed his frustration to EU ambassadors on Wednesday that the European Commission had pushed for a joint paper on the areas where there was convergence, but that Frost refused to give it political backing.
London is understood to believe that the time is not right for such a paper, given the divisions, but that ideas and papers have been "exchanged" by the British side.
Brussels sees the technical talks in terms of three distinct "baskets". The first deals with what the EU regards as the core sticking point, the movement of goods between GB and NI.
Here a senior official says the EU is suggesting practical solutions that would cut 80% of the agri-food checks and 50% of the customs formalities. The official said there was agreement between both sides that a distinction should be made between goods clearly destined for the south (ie, the internal market) and those clearly destined for Northern Ireland.
"The question is, how do you determine whether a good is destined for Northern Ireland or for the internal market?" the official says.
"What we think does not work is just to leave it to traders to say where it goes. We think that a sufficient amount of data needs to be collected in all cases, in order to then be able to do a credible risk assessment to work out whether the destination as declared actually makes sense or not."
The second basket covers issues that had not been raised as a problem by London until it published its Command Paper on 21 July, such as VAT and competition rules.
Under the protocol, Northern Ireland follows EU VAT rules for goods, but UK rules for services.
The commission argues that the Joint Committee, set up under the Withdrawal Agreement, already allows for both sides to discuss problems in the VAT area. "And yet the UK have never tried that," says the official.
The third basket covers "governance", or an enhanced role for Stormont and Northern Ireland stakeholders in the implementation of the protocol. Here the EU says the UK attempts to bring the ECJ into play, and things get no further.
In the House of Lords on Wednesday, repeating the UK’s ECJ demand, David Frost gave an equally tepid account of the talks and warned that if they did not meet the UK’s demands then he would have no alternative but to trigger Article 16.
The EU proposals were "worth discussing" but the gaps remained substantial. The UK was insisting that the "interlinked issues of the imposition of EU law and the [European] Court of Justice [on Northern Ireland], state aid, VAT, goods standards" had to be addressed in order to put the protocol "on a sustainable footing".
Frost challenged the EU’s claim that their proposals would cut customs formalities by 50pc. "They do not eliminate a single customs declaration for any good moving into Northern Ireland," he said. "The famous 50% figure is actually a 50% reduction in the number of fields in the customs declaration, with most of the significant ones still remaining."
An EU official later said the commission stood over its claims.
However, it’s understood that London still wants to work out how exactly the 80% and 50% cut in formalities, ie for sanitary and phytosanitary (SPS) and customs checks, would actually be delivered.
The EU’s overall position is that its October proposals represented a big move forward and the UK has still not reciprocated. David Frost believes the proposals are tinkering around the edges, and that a major renegotiation is needed to sustain unionist support, and thus to "rebalance" things towards the Good Friday Agreement.
The most sensitive issue is the ECJ.
"The UK’s position on governance is that the role of the European Union's institutions [in the Protocol] needs to go," says a senior EU official.
The EU insists the ECJ sits atop a broader governance structure, which London ultimately does not accept.
However, if Northern Ireland is in the single market for goods, then it must be part of that structure. "So long as that remains the UK's position, I don't see what we can do," says the EU official.
Is there room for manoeuvre?
So far there is little sign of it. UK sources suggest that London does not deny there has to be some role for the ECJ when it comes to matters of EU law, but that if there is a dispute between two sovereign entities - the UK and the EU - then the ultimate arbiter cannot be the EU’s court.
Some nuanced thinking may be detected on the UK side, which suggests London might be softening its line.
If both sides can agree arrangements where there is less friction on movements of goods between GB and NI, then surely there will be less need for disputes and arbitration anyway, is the thinking.
Another UK argument is that since the EU has agreed that GB sausages and chilled meats will be able to enter Northern Ireland - with some caveats - that means the EU ban on such goods entering the single market has already been variegated.
The reasoning continues that if further variations of EU law are agreed over time through the Joint Committee, then by definition there is already some friction with the ECJ, in that ECJ jurisprudence would, according to this rationale, be at odds with these developments.
London regards this as a gap that both sides should step into in order to discuss a less overt role for the ECJ.
So far there has been no discussion at technical level about this since the EU regards the removal of ECJ as the ultimate arbiter of EU law as a non-starter.
The overall mood in Brussels was already pessimistic this week.
On Monday, a prominent think-tank held a Chatham House rules meeting involving senior EU and UK officials and former ministers, policy experts, Northern Ireland business representatives and academics.
This was the latest in a number of informal gatherings going back two years designed to ensure an open exchange of views across the Channel, whatever the highs and lows of the formal EU-UK negotiations.
"The overall tenor was rather pessimistic given the divisions," says one participant, "including those along community lines in Northern Ireland. In terms of the way forward, it was difficult to see one, unless there's a move or an agreement between the EU and UK on how to move, and also an attempt by both sides to sell the protocol."
A senior EU figure made it clear at the meeting that if the UK was triggering Article 16 in order to force a wholesale renegotiation of the protocol, the EU response would be robust as it would be regarded as a breach of the Trade and Cooperation Agreement (TCA) and the sequencing approach.
There was another significant background encounter in Dublin. Julian Braithwaite, a senior British diplomat and director general of the UK Cabinet Office, held talks with senior Irish officials, as well as informal contacts with key figures.
While it was a long-scheduled visit, which had been postponed from some weeks back, the virulent atmosphere around the Article 16 threat dominated.
It’s understood Braithwaite may have been surprised by the strength of feeling about Article 16. The Irish Government message was that the talks should be allowed to succeed and there needed to be "constructive engagement" from London on the EU’s offer.
The Taoiseach’s warning that both EU-UK and Anglo-Irish relations would "suffer significantly" if Article 16 were triggered was repeated by officials.
"He was taken aback by how strong the reaction was," says a source briefed on the meeting. "He argued that the UK are engaged in the talks and working in good faith. The reaction from Dublin was, well Lord Frost at every conceivable opportunity mentions Article 16. So they shouldn't be surprised if we start taking them seriously."
On that note, David Frost warned on Wednesday that if the EU reacted in a "disproportionate way" to an Article 16 move, then the UK would no longer trust the EU’s "commitment to supporting the peace process and the best interests of the people of Northern Ireland".
Such rhetoric seems calculated to antagonise, leading many commentators to wonder what David Frost’s ultimate goal is.
UK sources insist the talks have some way to go, and have struck a less pessimistic note.
If so, London may be adjusting its position due to increased pressure from the EU, Dublin and, in particular Washington, where European Commission President Ursula von der Leyen met US President Joe Biden this week.
Downing Street could also face complications at home as well.
If the UK does trigger Article 16 because it wants a fundamental renegotiation of protocol, then Downing Street may well face a legal challenge in the UK courts.
That's because the protocol is written into British domestic law via the European Union (Withdrawal Agreement) Act.
The logic is that if London triggers Article 16 then ministers will want as much legal cover as possible.
Close observers of UK thinking point to a speech Frost made in the House of Lords on September 16, setting out how and where the UK would diverge from the EU’s legal orbit.
"Brexit is now a fact," Frost told the House of Lords. "This country is now embarked on a great voyage."
That voyage of divergence requires a change to British law so that the UK can ditch what is known as "retained EU law".
These are EU laws that operated in the UK before Brexit, but which were folded into British law after Brexit.
Frost told the Lords that retained EU law would no longer have special status as a distinct category of law because the UK wanted to diverge in certain areas, such as gene-editing in agriculture, artificial intelligence and clinical trials for new medicines.
If the UK wants to go in a different direction in whatever field, a new act of parliament would be required, so that ministers could adroitly change the UK’s approach through what is called "secondary legislation".
"Our intention is eventually to amend, to replace, or to repeal all that retained EU law that is not right for the UK," Frost said. "We will look at developing a tailored mechanism for accelerating the repeal or amendment of this retained EU law."
What does this have to do with Article 16?
It is understood Downing Street may want to use this new act of parliament not only to do away with retained EU law, but also to give ministers cover to get around the legal obligations of the Northern Ireland Protocol.
Section 7A(3) of the European Union (Withdrawal Agreement) Act makes it clear that any subsequent UK legislation must comply with the Withdrawal Agreement.
According to George Peretz QC, a respected commentator on UK and European law, the only way to get around this roadblock is for an entirely new act of parliament which would explicitly state that the UK intends to override section 7A.
"It has to be absolutely clear that in subsequent legislation that the government intends to breach the Withdrawal Act," says Peretz. "It's got to be absolutely clear and written in letters visible from outer space."
If the UK’s ultimate negotiating objectives are to make the Command Paper a reality, and the Command Paper rips up the protocol as is, then London would want to avoid a challenge in the domestic courts.
The Command Paper, after all, would completely rewrite the way in which goods move from Great Britain to Northern Ireland, a movement which at present is legally ordained by the protocol.
Without an act of parliament modifying Section 7A, a domestic legal challenge would be likely.
Article 16 states that the protocol must create "serious economic, societal or environmental difficulties that are liable to persist," or create the diversion of trade before it can be triggered.
Even then it must be triggered only for a limited period of time and with the least disruption to the operation of the protocol.
"Even a cursory reading of Article 16 shows that it is not the kind of provision designed to trigger renegotiation or even permanent adjustment of specific commitments or mechanisms in the protocol," wrote the noted US trade law expert Rob Howse in a paper for the DCU Brexit Institute this week.
There are a number of things to bear in mind.
The UK Attorney General Suella Braverman is already, according to the Financial Times, shopping around for legal advice on how to approach Article 16.
Indeed, Frost admitted this to the House of Lords on Wednesday: "We would want her to have the best possible advice, reflecting the full range of opinion on these very sensitive and unprecedented questions."
So, the decision to load a new Act of Parliament with powers to override the Withdrawal Act may not yet have been taken, and if it were it would face certain hostility in the House of Lords.
However, Frost is likely to want to be armed with as many legal tools as possible.
"Ministers tend to want all the legal weaponry they think is useful," says one source familiar with preparations. "They're likely to say, let's have these powers and then we can drop them as negotiations proceed, which was the Internal Market Bill approach."
On that score, the Internal Market Bill remained only that - a bill and not an act. The clauses overriding the Protocol were dropped when Maroš Šefcovic and Frost's predecessor Michael Gove reached an accord on implementing the protocol in December of last year.
While these manoeuvres - if that is what they are - suggest that Frost wants to armour plate his negotiating strategy, they still don’t explain the abrasive, confrontational style, in the face of growing EU determination not to give in, and even to tacitly threaten a trade war.
An analysis of Frost’s almost weekly speeches makes it clear that, despite his understated delivery, he is a very hardline Brexiteer who believes that the UK made a catastrophic blunder in 2017 by conceding that the least worst option in avoiding a hard border on the island of Ireland was for Northern Ireland to remain aligned with EU single market and customs rules.
By triggering Article 16, Frost may well believe that he could force the EU to rewind the clock to December 2017 and the Joint Report, the embryo of the current protocol.
In other words, in the post-trigger phase, which might last for up to a year before the TCA might be terminated or suspended, the UK would not carry out any controls on the Irish Sea, and then attempt to convince the EU that, look, there is little or no risk of unchecked goods entering the single market via the backdoor of Northern Ireland.
London may feel then emboldened to demand what the "alternative arrangements" of a hi-tech land border are and ask if the EU if it wants to put up a hard border on the island of Ireland then it’s up to them.
Frost would, in effect, want to test to destruction the theory that the EU will cave in such a scenario and move right over to the UK’s understanding of what the risk to the single market is. Frost almost certainly believes Boris Johnson did not have the courage to test that theory in October 2019.
"Don't forget," says one close observer of Frost’s approach, "he is the John Taylor, or even the Martin McGuinness of Brexit, the hardliners’ hardliner. He’s the guarantee that there won’t be a compromise, whereas Boris Johnson is a much more equivocal figure."
Of course, it will ultimately be Johnson’s call.
"If they’re still getting hammered on sleaze headlines and feeling really uncomfortable," says Paul McGrade, a former Foreign Office and European Commission official, currently senior counsel with the Lexington consultancy.
"And it’s pretty clear that COP hasn’t delivered, that's where the temptation might become real for Johnson. But he’s never gone that far before. The biggest risk is miscalculation."
The risks of miscalculation are by no means insignificant, and will likely play into the timing.
If Article 16 is triggered after COP26, then "rebalancing measures" from the EU could follow swiftly. France may decide to tighten controls on ferry traffic in and out of the UK. In the run up to Christmas, that could hit food supplies and supermarket shelves.
If the UK waits until February, when a putative new bill is ready, that might forestall a legal challenge, but would mean the instability and plummeting relations seeping into the Northern Ireland Assembly elections in May.
During Wednesday’s briefing by Šefcovic, a significant number of EU ambassadors said the response to a triggering of Article 16 should be "calm, methodical and proportionate", according to one source.
However, France (predictably) and Germany (less so) were both of the view that the response should be robust.
It has been nearly two years since the protocol was signed and in that time the UK has been either slow to comply, not ready to comply, non-compliant, or remorselessly hostile to it, notwithstanding the UK’s rationalisation of its position.
While member states have been content to let Maroš Šefcovic handle the relationship, observers believe the issue is starting to leak into domestic politics, in France, but also in Germany, where a new government is taking over.
Chancellor Merkel sought to dedramatize and be conciliatory, but her successor may well take a different approach.
"I suspect the new German government will take a harder line," says Fabien Zuleeg, of the European Policy Centre (EPC).
"It will be less tolerant of the UK trying to continue to weaken what has been agreed. [Incoming Chancellor] Scholz has a very good relationship with President Macron. There will be a lot of cooperation there, and certainly I don't think he will use any political capital to dissuade the French president from acting."
For the moment things are very delicately poised.
On Thursday, British Chancellor Rishi Sunak said the UK was "not yet" at the point of triggering Article 16.
Following their meeting on Friday in London, David Frost said talks would intensify next week, while Maroš Šefcovic said he acknowledged and welcomed "the change in tone of discussion with David Frost today – and I hope this will lead to tangible results for the people in Northern Ireland".
This may signal a last minute change of tack under pressure by London. But there have been many false dawns before.