Five and a half years after the UK voted for Brexit, getting on for two years since it left the EU and one year since the new world took effect, the subject that plagued the whole process is once again threatening to escalate into a major flashpoint.

       The row over Northern Ireland is the most serious issue straining EU-UK relations, even though the two sides struck a treaty on post-Brexit arrangements. Old arguments are being aired, old tensions revived.

       Brussels is offering concessions to ease the impact of the negotiated Northern Ireland Protocol but within the document's framework. London, which says it isn't working and effectively wants the agreement replaced, is unlikely to be satisfied.

       More discussions lie ahead but with the two sides far apart, it's hard to see how differences can be overcome and there are fears the row could degenerate into a trade war.

       Simon Usherwood, Professor of Politics at the Open University, says the current situation is "the latest in a long process of trying to find a stable balance between the EU and UK".

       "The former wants to try to make the treaties it signed just last year work, while the latter thinks there’s scope for some other solution, even if it’s not sure what that might be," he told Euronews.

       UK Brexit Minister David Frost offers EU 'new legal text' on Northern Ireland UK demands over Northern Ireland Protocol risk 'further breakdown in EU relations', Ireland warns

       What is the UK position?

       It's important to note that the protocol was not imposed on the UK — it was negotiated by both sides and at the time Boris Johnson praised an "excellent" deal.

       Its central thrust was that Northern Ireland would remain subject to some EU rules, and in order to avoid a hard border with the Irish Republic, internal UK trade between the province and Great Britain would need to be regulated.

       The UK Brexit minister David Frost said in his speech on Tuesday that the protocol was not working on the ground in Northern Ireland and was jeopardising the Belfast (Good Friday) Agreement — the 1998 peace accord that put an end to three decades of sectarian violence.

       Its impact has meant more bureaucracy — many products need certification — bringing some trade disruption between Britain and Northern Ireland, infuriating British unionists in the province. London has postponed indefinitely new border checks due to have come into effect.

       The protocol had "lost the consent of one community" in Northern Ireland, Frost said, repeating that the UK government may resort to triggering Article 16, suspending the agreement altogether.

       He also complained that the UK was being asked to apply a full boundary within its own country, apply EU law, and settle disputes in EU courts.

       Since this summer, the British government has raised objections to the role of the European Court of Justice (ECJ), as set out in the protocol to regulate Northern Ireland's continued adherence to EU single market rules on goods.

       "The constantly shifting focus of the problems identified by Lord Frost are really about avoiding a situation where the Irish Protocol can settle down and become the status quo: that could make any renegotiation even harder. The EU is well aware of this danger," Usherwood says.

       Brexit: EU's 'heavy-handed actions' have lost support for Northern Ireland protocol, says Lord Frost

       What is the EU's stance on the protocol?

       The EU argues that as part of the legally binding Brexit divorce deal, the UK should respect the protocol's terms. It takes the form of a carefully negotiated international treaty, dealing with delicate arrangements in a region that in the past was a renowned trouble spot.

       Throughout the process, the European Commission's stance has been backed by the national governments of member states.

       Brussels has been concerned at the UK's failure to apply the protocol, in particular by delaying checks and not installing infrastructure. It has also been frustrated at the UK's refusal to sign up to a veterinary agreement to ease bureaucracy.

       The Commission has however paused legal action against the UK and is now proposing measures to ease the protocol's impact. These are expected to involve a drastic reduction of the number of internal UK border checks, and special provisions for UK-only products — ending the so-called "sausage wars" that made headlines over the summer.

       But there has been no mention of altering the role of the European Court of Justice, which the UK has increasingly been playing up as a major sticking point.

       For Ireland and the EU, protecting the Good Friday Agreement is also of paramount importance. This means keeping the north-south border open, and protecting the integrity of the EU's Single Market and Ireland's place within it.

       "The Commission’s proposals are an attempt to short-circuit the British push, by offering as broad a package of easements and accommodations within the legal framework as possible," Simon Usherwood told Euronews.

       "The language about this being an offer for discussion is also important, as it prevents the UK treating it as a take-it-or-leave-it proposition, and thus grounds for more extreme action."

       Brexit 'sausage wars': EU plans 'dramatic' changes to ease Northern Ireland protocol row

       What is the impact on Northern Ireland — and on UK-EU relations?

       At best, the stalemate means more limbo for Northern Ireland when the province increasingly needs certainty.

       The impact of the protocol has been both practical in the form of trade disruption, and political. Businesses have experienced delays in getting supplies from Britain — where some firms have ceased supplying the province altogether — and some have looked to source products from Ireland.

       On the other hand, some firms say they have benefitted from the "best of both worlds", being part of the EU and UK internal markets.

       But the political impact has been explosive. British unionists oppose Northern Ireland being treated separately from the rest of the UK and argue the economic border threatens the union.

       The Democratic Unionist Party (DUP) — Northern Ireland's largest party — has threatened to pull out of the devolved government in the province unless the protocol is scrapped. This would almost certainly lead to its collapse.

       The British government's threat to suspend the protocol altogether, or at least its principal measures, presents a wider threat to the UK's overall relations with the EU. This would almost certainly lead to retaliatory measures and possibly a trade war.

       Even if the UK stops short of such action, Brussels is wary of an ongoing standoff where the protocol is not applied in full.

       "This is all very unsatisfactory, in terms of providing people in Northern Ireland with any certainty about arrangements, and it prevents the UK and EU from working together on other issues," says the Open University's Simon Usherwood.

       "However, London has evidently made the calculation that this disruption plays well enough for a domestic audience that the costs are worth bearing. The big question is whether this is sustainable: patience is rather thin and the possibility of things spiralling out of control is all too real."

       Northern Ireland: Change Brexit deal or we'll sink Belfast government, DUP warns Northern Ireland: Who's right in the EU-UK dispute over the Brexit protocol?

       How did it get to this?

       Dismantling the physical border infrastructure between north and south in Ireland, frequently the target of paramilitary attacks, was key to the 1998 peace accord after the Troubles.

       But Brexit brought a new headache: how to avoid a new hard border between Northern Ireland, part of the UK — and the Republic of Ireland, an EU member. Trying to resolve it bogged down the whole negotiation process, delayed Brexit itself, and brought down former prime minister Theresa May.

       Boris Johnson became prime minister in the summer of 2019, vowing to ditch the "backstop" measure — contained in the original version of the protocol as negotiated by May's government — arguing that this would have kept the UK tied to EU rules for years.

       Then in the autumn, almost overnight Johnson reached agreement with his Irish counterpart Leo Varadkar, and very quickly a deal with the EU in Brussels. The breakthrough secured him a much-sought general election, a big parliamentary majority, and delivered Brexit itself in that the UK finally left the EU in January 2020.

       However, it was obvious to many that the flip side of cracking the Brexit nut was an Irish Sea trade border between Northern Ireland and Britain.

       For some time, Johnson repeatedly and wrongly denied that this would involve internal UK checks and controls — despite the hard Brexit he was pursuing, with the UK outside the EU's Single Market and Customs Union.``

       Lord Frost argued in Tuesday's speech that "the UK’s negotiating hand was tied", citing "a widespread feeling in the UK that the EU did try to use Northern Ireland to encourage UK political forces to reverse the referendum result or at least to keep us closely aligned with the EU".

       But many in Brussels, around Europe and also in the UK, now strongly suspect the Johnson government of taking either a naive or a cynical approach to international treaty negotiations, recklessly ignoring the commitments and consequences this would bring.

       Post-Brexit Guide: Five years since UK vote, where are we now – and how did we get here?

       What they have said on the protocol row

       "The Protocol is not working. It has completely lost consent in one community in Northern Ireland. It is not doing the thing it was set up to do — protect the Belfast (Good Friday) Agreement. In fact it is doing the opposite. It has to change" — Lord David Frost, UK Brexit Minister, October 12.

       "UKG (UK government) creates a new “red line” barrier to progress, that they know EU can’t move on…. are we surprised? Real Q: Does UKG actually want an agreed way forward or a further breakdown in relations?" — Simon Coveney, Irish Foreign Minister, October 10.

       "We took over a party on ~10%, worst constitutional crisis in century, much of deep state angling for BINO or 2REF. So we wriggled thro with best option we cd & intended to get the (emoji of shopping cart) to ditch bits we didn't like after whacking Corbyn" — Dominic Cummings, ex-Downing Street adviser and 'Vote Leave' campaign guru on Twitter, October 12.

       "The EU's proposals for addressing the problems in NI are substantive & far reaching. They will effectively do away with all paperwork for goods destined for NI - instead of a border in Irish Sea, think of a “green” (NI-bound) & “red” (Single Market) lane" — Mujtaba Rahman, MD of Eurasia Group consultants, October 9.

       "The issue is not sausages, but sovereignty. The Government cannot allow the continued jurisdiction of the European Court of Justice over the NI Protocol" — Nick Timothy, former Chief-of-Staff to ex-Prime Minister Theresa May, October 10.

       "The absolute state of David Frost trashing the deal he negotiated + hailed as a triumph - despite many, yours truly included, warning it was a dud - *and worse* now using it to further undermine our relationship with some of our closest friends in an increasingly dangerous world" — Gavin Barwell, another ex-Chief-of-Staff to Theresa May, October 12.