Why is Ireland such a difficult issue in Brexit talks?

Barring a few high-profile interventions from two former prime ministers, Sir John Major and Tony Blair, Ireland was not the main issue in the EU referendum campaign. Yet the status of the 310-mile border is now the biggest stumbling block to a Brexit deal.

The EU and UK have pledged to avoid a hard border, including any infrastructure that could become a target for militants, in order to preserve the Good Friday agreement. But there has been little progress on how to avoid a border, since an initial agreement was struck in December 2017. Without an accord on Ireland, the EU has said there can be no Brexit divorce treaty. According to the EU logic of Brexit sequencing, without a withdrawal agreement, there can be no transition period or trade agreement between the UK and its European neighbours.

What is the EU plan?

Brussels wants guarantees it will be able to control goods flowing over the EU-UK border after Brexit. Without any controls, officials fear that it will be impossible to prevent illegal imports, such as American chlorinated chickens, or cheating on VAT rates. As the EU and UK are not expected to conclude an agreement on their future trading relationship until 2020 at the earliest, the EU has come up with a fallback plan, known as the backstop. The backstop means Northern Ireland would remain subject to the EU customs union and many single market rules, creating “a common regulatory area” on the island of Ireland, where goods can move freely. Northern Ireland would have to apply EU tariffs on non-EU goods. It would follow EU rules on product safety, animal welfare and VAT. EU institutions, including the European court of justice, would retain their role in enforcing these rules.

What is the UK plan?

The UK has staunchly rejected the EU plan: no British prime minister “could ever agree” to the backstop, which threatens the constitutional integrity of the UK, Theresa May has said. The proposal is especially fraught for the prime minister, whose minority government is propped up by the Democratic Unionist party, which fiercely opposes separate status for Northern Ireland.

The UK wants to resolve the Irish question through an agreement on the future trading relationship. It argues that any fallback plan should apply to the whole of the UK and be time-limited, two points the EU refuses to accept. The UK has proposed two customs plans. The first, “maximum facilitation”, is based on technology and trusted-trader status for small businesses doing cross-border trade. The second is an unprecedented partnership that would allow the UK to collect tariffs on behalf of the EU. Both ideas have been rejected by the EU, but elements have been reincarnated in May’s Chequers plan.

What happens now?

The EU hopes to convince the UK that its backstop is not trampling on British sovereignty. This is Michel Barnier’s attempt to “dedramatise” the issue, partly by appealing to precedents – existing Northern Ireland-only rules on phytosanitary (plant health) norms, controls between Spain and the Canary Islands. Barnier, the EU’s chief Brexit negotiator, is also said to be looking at allowing British officials to check goods coming to Northern Ireland. Another EU idea is whether checks could be done at ports and airports away from the land border. But British officials are said to be unconvinced. To the British, the EU proposals still look like an “unacceptable” border in the Irish Sea.

EU leaders are likely to signal the importance of solving the Irish question when they discuss Brexit at a summit in Salzburg this week. But a solution remains elusive.