Halt: why post-Brexit border posts could put a stop to business
The prospect of Brexit has already caused deep division in Northern Ireland between parties, with leaders taking different sides on whether Britain should leave the EU.
Nationalist politicians and many business interests fear the reinstatement of a "hard border", with customs checks and tariffs delaying the free flow of goods and people.
First Minister Arlene Foster of the DUP wants the UK to leave the EU, while the Deputy First Minister Sinn Féin's Martin McGuinness has taken the side of David Cameron and is in favour of staying.
Adding to the controversy is the stance of Northern Ireland Secretary Theresa Villiers, who is campaigning for Brexit against the wishes of her own leader. She also appears to be out of step with public opinion in the North, which is against leaving the EU, according to recent opinion polls.
McGuinness has called on Villiers to resign following her public support for Brexit.
"Isn't it odd that Theresa Villiers should now be advocating withdrawal from the EU when clearly the benefits for the North are very, very obvious," McGuinness said.
Villiers dismissed calls for her to step down from her Cabinet position while campaigning for a Leave vote, telling the BBC: "I think it's perfectly reasonable for me to have chosen a side in this referendum. The great thing is that every single person in the UK, including in Northern Ireland, will get to take this decision, not just the Secretary of State."
Asked whether Brexit would mean tighter border controls with the South, Villiers said: "That's not inevitable at all. We've always had a much closer relationship with the citizens of the Republic of Ireland than with the rest of the EU."
However, McGuinness said the implications of a Brexit would be "absolutely enormous".
"Our land border with the Republic could be a major issue, given the desire in Europe to tighten borders."
The First Minister also questioned how a withdrawal would affect cross-border agreements made during the peace process.
Since 1995, the EU has paid up to €2bn to support peace initiatives in Northern Ireland and the border counties in the South.
It has helped build a peace bridge linking mainly Catholic and Protestant communities in Derry, and the Skainos Centre in East Belfast.
The latest pot of European money, €537m, was announced last month and applications have already begun. The programme is due to end in 2020.
While Villiers is confident that there will not be tighter border controls between North and South, it is hard to see how the border could function without some kind of checks if Britain was outside the EU.
In the past, those travelling with goods across the border were stopped at customs posts and also had to negotiate security barriers. Many roads between the North and South were also blocked.
There were frequent queues of lorries at the border, and those travelling through could easily be delayed by up to an hour.
A Big100 poll for the Belfast Telegraph earlier this month showed that 55pc of voters in the North are in favour of staying in the EU, and 29.5pc are in favour of leaving.
The result of the referendum in June will not only have implications for Britain's status in the EU, but also the constitutional future of Northern Ireland and Scotland.
If the UK overall votes to leave the EU, but the votes in Northern Ireland and Scotland favour staying in, it is likely to cause instability. The Scottish Nationalists have indicated that if Britain votes to leave, they will demand another referendum for independence, and that could have a knock-on effect in Northern Ireland.