Fears are growing that the UK will trigger article 16 of the Northern Ireland protocol, potentially rupturing the already strained relations with the EU. If talks with the EU collapse, it is expected just after Cop26 in seven days’ time. On Wednesday Ireland’s prime minister heightened concerns that this was almost a racing certainty after his bluntest warning yet to the UK that such a move would be “reckless”, “irresponsible” and have “far-reaching consequences”. What is article 16? It is a clause in the Northern Ireland protocol, one of the key elements in the withdrawal agreement, which legally cemented the UK’s departure from the EU in January 2020. It allows either side to take steps, or “safeguards” in treaty-speak, if the protocol leads to “serious economic, societal or environmental difficulties that are liable to persist, or to diversion of trade”. What is the test for triggering article 16? The UK must prove that the protocol has given rise to “serious economic, societal or environmental” difficulties. Lord Frost has repeatedly said the conditions to trigger it are met because of the obstruction to trade flow between Great Britain and Northern Ireland. What are these safeguards? They have not been defined but they must be “appropriate” to the damage caused and are likely to include the semi-permanent suspension of checks on goods flowing from Great Britain to Northern Ireland. But the invoking of article 16 will not have a huge impact on the ground. This is because many of the checks have already been unilaterally suspended by the UK and the article 16 process itself triggers a formal dispute process that requires both sides to go into talks to resolve the dispute. So why all the fuss? The fear is the UK will go much, much further and use the breakdown of talks to drive through domestic legislation to dump the role of the European court of justice in the arbitration process, something that cannot be achieved through article 16. This would be a sequel to September 2020 when the government admitted it was going to “breach international law” in a “specific and limited way” through the internal market bill. That move was eventually abandoned but not before the resignation of the UK’s top legal civil servant, Jonathan Jones, and a mutiny in the House of Lords by a gang of former Tory party leaders. The Brexit minister, Lord Frost, has repeatedly made the ECJ a red line so many expect that triggering article 16 will be involve multiple connected actions. As one Whitehall source said “Frost is looking for a time machine” trying to undo Theresa May’s 2017 deal which embroidered the ECJ into the Brexit deal. Will there be a trade war? The EU could not retaliate immediately under the arbitration process. However, it has gameplanned various scenarios and many believe a move to abandon the ECJ through domestic legislation would be tantamount to the UK walking away from the international withdrawal agreement. So the language used by both sides will indicate whether the UK and the EU are ready to go on a trade war footing. Can a wider trade war really be triggered by a breakdown in talks over Northern Ireland? Yes. Remember when the EU threatened to trigger article 16 during the row over vaccine supplies in January? What came as a shock to many commentators was the fact that measures in the Brexit protocol designed to protect peace in Northern Ireland could be deployed in a completely different theatre of war. Some are already suggesting the EU could announce its plans to suspend the trade agreement altogether. But under the rules the EU’s response has to be proportionate. Cambridge University EU law professor Catherine Barnard says it would be in its powers to slap tariffs on British products such as Scottish salmon and whisky. This is what happened in the row with the US over steel and other products. The EU put a 56% tariff on Harley-Davidson bikes imported from the US with other countries imposing levies on bourbon, peanut butter and jeans, products that mattered in Trump-supporting states. Anything else? David Frost does not have the last word. A decision on article 16 is ultimately Boris Johnson’s. One Whitehall source said the fishing row had “focused minds on the impact on consumers” after France threatened to check every lorry coming into Calais and other ports. If France, Ireland and the Benelux countries decided to stop every lorry for Brexit checks, supply chains before Christmas would be severely disrupted, something that may weigh on the prime minister’s mind.