The NI executive should make staying in the EU customs union a post-Brexit priority, a think tank has said.
The UK will have to decide if it leaves the customs union when it leaves the EU.
That would mean goods moving between the UK and the EU would be subject to customs procedures and tariffs.
The Nevin Economic Research Institute (Neri) said leaving the customs union posed a "unique threat" to Irish cross-border trade.
It said it is likely to have a more immediate impact on the Northern Ireland economy than leaving the EU single market.
What is a customs union and why does it matter?
A customs union is a form of trade agreement between two or more countries.
It means they decide not to impose tariffs (taxes on imports) on each other's goods and agree to impose common external tariffs on goods from countries outside their customs union.
Setting common external tariffs is what distinguishes a customs union from a free trade area.
The key argument for leaving the customs union is that it will allow the UK to negotiate its own trade agreements.
However, Neri said that needs to be weighed against the potential damage to existing trading arrangements.
Neri said the executive could even consider trying to remain within the customs union even if the rest of the UK leaves.
That would mean customs checks on goods moving between Northern Ireland and the rest of the UK.
Neri said such checks at sea ports or airports are "less cumbersome or trade inhibiting" than those on land borders.
However, it acknowledged that such a proposal would have "significant political problems".
The executive's Brexit priorities were set out in a letter that Northern Ireland's first and deputy first ministers sent to the prime minister in August.
It said that Northern Ireland needs to "retain as far as possible" the ease with which it currently trades with EU member states.