LONDON (BLOOMBERG) - How to avoid a hard border between the Republic of Ireland and Northern Ireland has long been the sticking point in Brexit talks.
But as negotiations reach the endgame, it's become a high-stakes fight that could shape Britain's relationship with the European Union - and leave Britain shackled to the bloc's rules indefinitely. Pro- and anti-Brexit lawmakers are united in the opposition to such an outcome.
1. WHERE DO NEGOTIATIONS STAND?
The border problem arises in the first place because the UK wants to leave the EU's single market and customs union - two frameworks that allow the current frontier between Northern Ireland and the Irish Republic to be all but invisible. Both sides committed to avoiding a hard border after Brexit, and there can be no divorce deal unless that's upheld.
So the UK and EU agreed to include a guarantee clause - known as a backstop - in the divorce agreement. But the EU's proposal for how the backstop would work essentially means a border between Northern Ireland and mainland Britain. That was unacceptable to the UK.
London then made its own proposal to keep the whole UK in a customs union as a temporary measure. The EU is willing to accept this - but the conditions threaten to become a deal breaker.
2. WHAT'S THE NEW ROW ABOUT?
The EU is demanding that in return for accepting the UK's backstop, the UK will have to accept a bunch of EU regulation - not just on customs but also on things like state aid, competition and environmental standards. It's also not clear how the UK will be able to extricate itself from the backstop. The UK is demanding some kind of exit clause, but the EU has said it can't be unilateral.
First it was just the hardline Brexit-backers who objected. But now pro-EU politicians are also looking again at the small print. Jo Johnson, a pro-EU Conservative, resigned last week because he said the terms amounted to "vassalage".
3. WHAT'S THE FIGHT ABOUT THE LEGAL ADVICE?
Before she takes a decision at the negotiating table, Prime Minister Theresa May will take into account the legal advice she receives from Attorney-General Geoffrey Cox. The key question is whether the UK will get trapped in the backstop, tied to EU rules.
Labour lawmakers and hardline Brexiteers are trying to force May to publish the advice, in the hope it will confirm their suspicions and oblige her to change tack. There's a vote on it on Tuesday (Nov 13) in Parliament, and Labour is using an arcane device it's deployed successfully before to get the papers into the open. Legal advice isn't usually made public.
Meanwhile Brexiteers are trying to draw parallels with the Iraq war - former prime minister Tony Blair didn't publish legal advice beforehand and it's still the subject of debate 15 years later.
4. SO WHAT ARE MAY'S CHOICES?
May could commit to staying in a customs union, which the UK could only leave by mutual agreement. This risks leaving it bound to EU rules, and would be seen as a betrayal by Brexiteers in May's party. There are also pro-EU lawmakers who don't like the small print.
But to further complicate matters, the plan will also enrage the Northern Irish Democratic Unionist Party, which prop up May's government, because it would still involve additional checks on goods moving between mainland Britain and Northern Ireland.
If the UK refuses, then the alternative could be accepting the EU's backstop, which May has long rejected as it would carve Northern Ireland into a separate customs territory to Britain. That might deliver what Brexit purists want, though it would come at the cost of allowing the EU to keep Northern Ireland inside the bloc's customs territory even as mainland Britain goes a separate way. Again, this would enrage May's Northern Irish political backers - and she needs their votes to get her deal approved in Parliament.