A hard border could be created around Northern Ireland after Brexit even if the UK initially agrees a soft border with the EU, MPs have been told.
Economist Paul Mac Flynn gave the assessment during a hearing of expert witnesses by a committee in Westminster about the impact of Brexit on the Irish border.
Mr Mac Flynn raised concerns about proposals previously outlined by the UK that only large businesses rather than small or medium ones could be required to register their imports and exports along a post-Brexit border, saying people could find ways to get around such rules.
He told MPs : "I think the danger is, we could start out with a soft border and end up with a hard border. Like the exemption for small and medium sized enterprises.
"The idea was first, that they were going to say that for 6% to 8% of heavy goods vehicles.
"If you're somebody who is going to want to get around a tariff border, all that says to you is - right, don't use trucks, use vans. Then we start checking vans, so we say we'll start using state cars. Before you know it, we're checking everyone."
Mr Mac Flynn, who is a senior economist for the Nevin Economic Research Institute, added: "I think it has to come down to this - if you're leaving the customs union, that has an implication for what the border in Northern Ireland is going to look like. Say that one is being sacrificed to the other, but at least be honest about it."
MPs on the Northern Ireland Affairs Committee heard evidence from witnesses on a range of issues as part of an ongoing inquiry into how Northern Ireland "faces a unique set of challenges resulting from Brexit" due to sharing a land border with the Republic of Ireland.
Concerns have been raised that if the UK leaves the customs union or the single market, this will result in checks being erected along the border with Ireland in order to regulate flow of goods.
EU negotiators have said the border with Ireland is one of the key issues they seek to address early in Brexit negotiations in order to progress on to further stages of talks.
In August, the UK published a position paper on Northern Ireland and Brexit.
However, in September EU chief Brexit negotiator Michel Barnier said he was "worried" by progress on the matter and called on the UK to do more to come up with solutions.