Britain's border with Ireland was almost completely absent from the debate in the run-up to the country's 2016 vote to leave the EU. But the 310-mile (499-kilometer) frontier is now at the heart of the Brexit saga.
The backstop is strongly supported by the European Union — and the Irish government — but is deeply unpopular with many in Prime Minister Theresa May's Conservative party. So what is the so-called backstop and why has it become so controversial?
At present, goods and services are traded between the UK and Ireland within very few restrictions because both are members of the EU single market and customs union. A hard border would mean that there would be physical checkpoints for customs inspections and charges, monitoring people and goods crossing in and out of the UK.
The backstop is a safety net to ensure an an open border between Northern Ireland and the Republic of Ireland in the event that the UK leaves the EU without securing a deal. Northern Ireland would stay in the customs union and much of the single market, guaranteeing a friction-free border with the Republic.
The UK and Ireland are currently part of the EU single market and customs union, so products do not need to be inspected for customs and standards. But, after Brexit, all that could change — the two parts of Ireland could be in different customs and regulatory regimes, which could mean products being checked at the border.
Read more: The Irish border — what you need to know
British, Irish and European politicians have all voiced their opposition to any hard border in Ireland in the two-and-a-half years since the UK voted to leave the EU. In July 2016, a month after the Brexit vote, May declared that there would be "no return to the borders of the past," promising a "practical solution" to the border.
But finding a solution that would ensure that there would be no customs posts or physical barriers on a frontier that often witnessed violence during Northern Ireland's "troubles" (1969-1998) has proven far harder. The backstop is intended to be that solution.
Will the backstop do the trick?
In December 2017, May and the European Commission agreed that a mechanism to prevent a hard border in Ireland was needed in the event that a plan was not agreed between both sides. The backstop would immediately come into force if no trade deal was agreed after the end of the Brexit transition period.
In a foreshadow of May's current problems, her then Brexit Minister David Davis said once the agreement was signed the UK should have the power to "unilaterally decide when it considers the backstop has served its purpose."
The agreed plan would see Northern Ireland staying aligned to some rules of the EU single market, if the backstop was implemented, which would mean goods travelling into Northern Ireland would need to be checked to see if they meet EU standards. The plan also involves a temporary single custom territory which essentially keeps the whole of the UK in the EU customs union. Crucially, this will remain the case unless both the EU and UK agree that it is no longer necessary.
Read more: Brexit: What Europe wants
"If the backstop kicks in it basically means that Northern Ireland will be part of the single market for certain specific goods, mainly animal products and food. The EU are saying we will treat what comes from Northern Ireland as coming from single market with the UK making sure that Northern Ireland is meeting European standards," Dr Katy Hayward, a sociologist at Queen's University, Belfast and an expert on the border, told DW.
Caught up in a customs union
The backstop has proved wildly unpopular Brexit supporters, who are concerned that the UK could end up in an "open-ended" customs union arrangement, despite attempted reassurances that the backstop is intended "to apply only temporarily." The Democratic Unionist Party — a small Northern Irish party upon who May relies for her parliamentary majority — and Conservatives in the pro-Brexit European Research Group both voted against the prime minister's deal.
Conservative critics say that technology could solve the Irish border, with proposals including moving customs checks away from the frontier. But Hayward says such an approach is not possible.
"Their argument is you can do it all away from the border, that it can be solved by technology. But while technology has been sped up, that is all about the information you give prior to crossing the border so would massively increase the amount of paperwork firms needed to process."
A system for obtaining prior customs approval does exist, says Hayward, but "it's really meant for large companies. It's not appropriate for small businesses trading across the Irish border." The border region is the poorest in Ireland, and many of the firms that trade across the frontier are smaller businesses.
No deal plans
As concerns about the UK leaving the EU without a deal have grown, the Irish government has stepped up its no deal planning. A recent contingency plan called for new checks at airports and seaports on goods if Britain leaves without a deal. The measures include customs inspections, agricultural checks on meat shipments and health checks on non-animal foods.
The Irish government has said that it would be "very difficult" to avoid border infrastructure in Ireland under a no-deal Brexit, but has refused to consider any changes to the backstop arrangement in the current withdrawal agreement. It also says that it will not reintroduce checks at the border itself. But experts believe that some form of custom controls would be inevitable if no agreement is found between the EU and the UK.
"The rest of the EU is going to say: 'we don't want Ireland being a weak spot in the single market.' The EU is really strict about customs controls at its external border, so it wouldn't tolerate it for long," said Hayward.