Why Irish border deal is crucial to Brexit

The lack of an agreement on the Irish border has become a formidable stumbling block to the progress of Brexit talks.

WHY THE IRISH BORDER IS CRUCIAL IN BREXIT TALKS:

* Invisible border

With Northern Ireland leaving the EU along with the UK, it creates a 500km land border with the Republic of Ireland, which will remain a member of the EU

The border is invisible at present: almost 300 crossing points have no customs posts or border infrastructure, and thousands of people live on one side and work, shop or go to school on the other. This is possible because both the UK and Ireland are part of the EU's borderless single market for goods and services as well as the tariff-less customs union.

The British government says that when it leaves the EU in March 2019, it will also leave the single market and the customs union.

Ireland and the other EU members are demanding the UK provide details of how customs checkpoints and other border obstacles can be avoided before negotiations can move on to their next phase of discussing post-Brexit relations like trade.

* Impact of a hard border

Businesses on both sides of the Irish border have grown used to operating in an "all-island economy" in which goods and workers flow freely. Northern Ireland dairy farmers ship milk to the south to be processed. Ireland's famous Guinness stout goes north to Belfast for bottling. Anything that makes that seamless movement harder, whether customs checks or new tariffs, could have a major impact on trade.

* What is the solution

One solution is to allow Northern Ireland to stay in the customs union when the rest of the UK leaves. But it is fiercely opposed by Northern Ireland's Democratic Unionist Party and Theresa May's minority government relies on DUP support to stay in power.

An agreement that would commit Britain to maintaining "regulatory alignment" between Northern Ireland and Ireland after Brexit would make it easier to avoid a hard border but limit the UK's ability to change its economic regulations to strike new trade deals across the world.