Q&A: The problem with the no-deal border plan?
The former head of the UK government's Border Delivery Group has said the no deal plan for the Northern Ireland border is unsustainable for more than a matter of months.
BBC News NI's John Campbell looks at the plan.
What's the problem?
If the UK leaves the EU without a deal, Northern Ireland and the Republic of Ireland will immediately be in different customs and regulatory territories.
Normally, in such a situation, countries will carry out customs procedures and other checks at their borders to prevent smuggling.
This involves customs posts and other infrastructure. Even Sweden and Norway, which are both in the EU's single market, have customs checks at major border crossings.
However, the UK government has promised that it will not harden the Irish border in any circumstances.
What's its solution?
The UK said it will not impose tariffs on Irish goods crossing the border into Northern Ireland.
In addition, it will not introduce any new checks or controls on goods at the land border.
For Irish exporters to Northern Ireland, it would be like Brexit hasn't happened.
However, Irish goods going direct to Great Britain, such as on the Dublin-Holyhead route, will be subject to new tariffs and controls.
So won't Irish exporters just ship their GB exports through Northern Ireland to avoid tariffs?
To some extent this will be possible.
HM Revenue and Customs (HMRC) say there will be a "general anti-avoidance rule" to discourage Irish firms from using Northern Ireland as a tariff-free back door into GB.
However, if Irish businesses have traditionally shipped through Northern Ireland, they can continue to use this route without facing tariffs.
If they begin to ship products on this route for genuine commercial reasons, other than avoiding tariffs, that will also be permitted.
Irish products which are processed in Northern Ireland and shipped onwards into GB will also be tariff free.
HMRC will not be applying any new checks or controls on products crossing from Northern Ireland ports to GB. Instead, there will be "intelligence led" enforcement.
What about products going from Northern Ireland to the Republic?
The no tariff, no checks plan is a unilateral initiative by the UK - it will not be matched by Ireland.
As an EU member, Ireland will be obliged to impose tariffs on Northern Ireland goods crossing the border and carry out checks to protect the single market.
In effect, this means Northern Ireland goods, particularly in the agrifood sector, will face substantial new trade barriers but Irish goods going in the other direction will not.
The government has acknowledged this will impact on the competitiveness of Northern Ireland businesses.
The Ulster Farmers' Union said that in order to protect the food industry in Northern Ireland the plan should be changed.
The union's president, Ivor Ferguson, said the current plan would mean a "catastrophic" outcome for farming and food production in Northern Ireland.
"Steep export tariffs, additional checks and regulations, combined with a proposed zero tariff on agricultural goods from Ireland to NI, will result in significant disruption and pose a logistical nightmare for farm businesses."
Does the plan comply with World Trade Organisation rules?
The most important WTO principle is that members do not discriminate. So, if zero tariffs are applied to one member, they should be applied to all other members.
At a glance, the UK government's plan for cross border trade with Ireland would appear to conflict with that.
The government is confident that the arrangement will not break WTO rules and that various exemptions to the non-discrimination principle could apply.