Nearly six months since the UK's new life outside the EU's orbit began for real, one issue in particular continues to poison relations, just as it plagued the tortuous divorce process: Northern Ireland.
At the heart of the dispute is the Northern Ireland Protocol — agreed by the EU and the UK as part of the binding Brexit divorce treaty. It deals with a territory which is part of the United Kingdom but has to keep an open land border with the Republic of Ireland, an EU member. Economic ties and rival allegiances stretch both east to Britain, and south to Dublin.
The protocol keeps Northern Ireland subject to many European Union rules. This has had the effect of creating an internal UK trade barrier with England, Scotland and Wales. Its practical effect so far has brought disruption to supplies and sparked anger among UK unionists.
The EU says the UK should respect the terms it signed up to. The British government argues the current situation is unsustainable and accuses the EU of inflexibility.
What's happened since the protocol came into force?
The new measures took effect on January 1, 2021, as the UK left the EU's Single Market and Customs Union.
During the first few weeks of the year, Northern Ireland experienced significant disruption to supplies from Great Britain, particularly of food products. There were shortages in supermarkets, and also in garden centres whose products remain subject to EU plant health rules. Some stores started sourcing supplies from elsewhere, including the South.
Such problems brought an angry reaction among unionist communities and even contributed to a spate of rioting in April. Northern Ireland businesses said some firms in Britain had stopped supplying them.
A three-month grace period was agreed exempting supermarkets from checks on goods entering from Britain. But when the UK government decided unilaterally to extend the suspension on agri-food controls until 1 October, the European Commission called the move "a violation" of Brexit agreements and launched legal action.
A separate grace period due to expire on July 1 concerns chilled meats, which won't then be allowed into Northern Ireland from Britain unless the two sides strike a deal. Amid what was dubbed a "sausage war" in recent weeks, the UK threatened to extend this period unilaterally. Agreement is likely, but underlying issues remain unresolved.
The political repercussions from the new arrangements have seen turmoil in Northern Ireland's largest political party. The Democratic Unionist Party (DUP) is on its third leader in as many months since Arlene Foster announced her resignation in April.
What does the protocol put in place?
The protocol was negotiated against a background of Northern Ireland's troubled recent past, where decades of sectarian conflict ended with the Belfast (Good Friday) Agreement. But Brexit has put the 1998 accord under stress: its measures were anchored in common British and Irish membership of the EU — which of course is no longer the case.
The treaty tries to reconcile the UK's freedom to diverge from EU rules and standards and pursue an independent trade policy, with the need to control goods entering the EU's Single Market — while respecting the political imperative of keeping an open land border on the island of Ireland.
Unlike the rest of the UK, Northern Ireland continues to follow some EU rules — which effectively creates a regulatory border with Great Britain across the Irish Sea. The EU imposes checks and restrictions on goods entering from non-EU countries.
The protocol states that with Brexit, Northern Ireland leaves the EU's Customs Union along with the rest of the UK and is guaranteed "unfettered access" to the UK's internal market. There are no EU tariffs on goods sent from Britain unless they are "at risk" of entering the EU, or on personal or exempted goods entering the Irish Republic.
However, Northern Ireland still follows EU customs rules, remains part of the Single Market for goods, applies EU law on VAT (Value Added Tax), and is subject to EU oversight.
Most goods entering Northern Ireland from Britain need a customs declaration. New control posts have been set up at ports, where regulatory checks on goods including some food and plant products sent from Britain are carried out.
A separate agreement was reached in December 2020 on the protocol's implementation, with oversight from a EU-UK joint committee.
What solutions are being proposed?
Some Northern Ireland unionists argue that the protocol — or the way it is being implemented — breaches the Belfast Agreement by imposing an internal UK border. The DUP's new leader, Sir Jeffrey Donaldson, has warned of harmful political and economic consequences if the protocol is not revised.
"There is a commitment in NDNA to remove barriers to trade between GB & NI created by the Protocol & this is hugely important to unionists," he tweeted last weekend, referring to the "New Decade, New Approach" agreement which restored the devolved government in Northern Ireland in 2020.
The UK's Brexit minister David Frost has called for "pragmatic solutions" and more flexibility from the EU over the protocol.
Negotiations between London and Brussels on its application are ongoing. The European Commission is considering the UK's request for an extension to the grace period on chilled meats, which is expected to be granted.
But business groups have warned that permanent solutions are needed to overcome red tape, such as a "trusted trader scheme" and a veterinary agreement on meat and dairy products.
The EU says the UK's refusal to accept a Swiss-style deal on veterinary arrangements and food safety standards — where the UK would continue to follow EU rules — is a missed opportunity to slash many checks.
But the UK argues that the ability to diverge from EU norms is the essence of Brexit. The government's own proposals are based on "equivalence" where each side would recognise the other's high standards.
Officials in Brussels deny the EU has been inflexible, pointing to efforts made to find solutions to ease the movement of medicines, guide dogs and reduce red tape concerning live animals. They accuse the UK of dragging its feet in implementing new procedures — which London denies — and EU leaders have urged the British government to respect the terms of the accord.
European Commission President Ursula von der Leyen told Euronews she wanted the British "just to do their job and implement what we have agreed upon". French President Emmanuel Macron said: "Prime Minister Johnson... himself signed the agreement protocol that is applicable for Northern Ireland which lays out controls".
How consistent has been Boris Johnson's stance?
At the G7 summit earlier in June, Boris Johnson suggested that the EU was being "theologically draconian" over the protocol's application. He added that some European leaders seemed not to understand "that the UK is a single country, a single territory".
But the treaty he negotiated refers more than once to "the unique circumstances on the island of Ireland", setting out "arrangements necessary" to address this. An extensive annex lists numerous EU laws still to apply in Northern Ireland. There are many provisions dealing with its particular economic position related to Brexit.
The protocol in its current form resulted from Boris Johnson's agreement struck with the EU in October 2019. This broke the long deadlock over Northern Ireland that had delayed Brexit itself. It paved the way for the divorce deal with the EU, a general election victory, and finally the UK's exit from the EU in January 2020.
The prime minister's deal — he called it "excellent" at the time — did away with his predecessor Theresa May's ill-fated "backstop" measure, which his wing of the Conservative Party had strongly opposed, arguing it could have kept the UK in the EU for years.
But the new accord accepted Northern Ireland's special status compared to rest of the UK — and effectively created an Irish Sea border with Britain.
Over the ensuing weeks, Boris Johnson repeatedly and wrongly asserted — examples are here and here — that there would be "no checks" on trade between Northern Ireland and Britain in either direction, contrary to the terms of the divorce deal he had just struck. Ministers later accepted that the protocol did involve internal UK red tape.
Speaking about the protocol last week, former Conservative minister for Northern Ireland Chris Patten said: "The problem at heart is not the sausages you get from Sainsbury’s but the porkies that we all get, home and abroad, from Downing Street."
Can the protocol be suspended or annulled?
"If the protocol continues to be applied in this way, then we will obviously not hesitate to invoke Article 16," Boris Johnson said on the sidelines of the G7 summit. He was referring to an emergency measure in the protocol allowing either side to suspend parts of their agreement.
The European Commission briefly proposed invoking it in January amid a row over coronavirus vaccine exports, to stop doses from Ireland crossing the border. It quickly ditched the plan and recognised the move was a mistake.
Article 16 says that if the protocol's application leads to "serious economic" or other difficulties, the EU or the UK "may unilaterally take appropriate safeguard measures". But these must be limited and the other party can take "rebalancing measures".
An annex sets out a procedure to be followed. Even if one side is thinking of taking such a move it must notify the other, enter into consultations to try to find a solution, and not act for a month. The measure is clearly intended as a last resort.
There is another path for the protocol to cease to apply: Northern Ireland's assembly will be able to decide on whether to keep the new arrangements — but only four years after the transition period.
How serious are the consequences of the protocol dispute?
Despite eventually striking two major Brexit deals — on the divorce terms, and then on future trade and cooperation — EU-UK relations remain antagonistic six months on from the end of the transition period.
London and Brussels have clashed over fishing rights, diplomatic representation, coronavirus vaccine exports — but it is the row over Northern Ireland that poses the most serious threat to future relations, and perhaps even to the future of the British union.
Some commentators have noted that relations have become worse since the UK's post-Brexit negotiations with the EU were taken over by David Frost, now the UK's Brexit minister.
David Henig, director of the trade think tank ECIPE, argues that the British stance on the Northern Ireland Protocol amounts to a rejection of global trade norms.
"Frost and advisors have come to believe that the (EU) single market is overly legalistic and there is an alternative, equivalence, that can prevent checks. Major problem is this doesn't exist anywhere in the world," he tweeted.
Boris Johnson's government continues to argue that the way the protocol is being implemented does not correspond to the spirit in which it was negotiated. Ministers accuse the EU of "legal purism".
But others point out that the prime minister struck his deal in the autumn of 2019 at a time when he was desperate to break the long Brexit deadlock and fulfil his vow that the UK would leave the EU at the end of October (in the end it left the following January). Were inconvenient details inadvertently — or deliberately — glossed over?
The conflicting stances and ongoing tensions on Northern Ireland suggest a fundamental clash in approach to trade, and international treaties, that does not bode well for future relations.