A Brexit would mean tariffs that would make trade less competitive and cost jobs, the boss of a fourth-generation Londonderry manufacturing firm that sends the bulk of its exports to the Republic has claimed.
Paul Diamond, managing director of packaging company Diamond, which employs 80 people, said an exit from the EU could also mean his firm putting off or reconsidering investment.
Mr Diamond added a vote to leave was a vote for uncertainty.
"We manufacture corrugated packaging, for example, supermarket shelf-ready packaging," he explained.
"We supply people in the food and drink sector, pharmaceutical, aerospace, e-commerce and general manufacturing.
"We supply throughout Ireland, and two-thirds is sold into the Republic.
"Realistically, no one really knows what will happen. If you vote to leave, you are voting for uncertainty.
"We are also working on the next stage of investment, and obviously if the vote is to leave we will have to weigh how that might affect our business.
"All businesses are in the exact same position. We don't know what is going to happen."
The firm has just invested almost £6m, which included putting money into Europe's most advanced flexographic printer.
Mr Diamond said there was a "lack of clarity" on both sides of the Brexit debate.
"We are unique in the UK in the fact we share a border with the Irish Republic," he added. "This would leave us potentially less competitive. If we are less competitive, that means businesses shrink and that means jobs will go. That's the reality.
"If there are tariffs and some sort of cost applied, it will be applied to exports.
"Like most exporters, manufacturers work in a very competitive environment. If there is going to be an additional cost, it may leave us less competitive, and that's the fear we have. It's crucial that we get a remain vote.
"The only thing that really matters from a business perspective is the economy. We need to bring jobs."
He also claimed that in the worst case scenario, any tariffs applied between Northern Ireland and the Republic would leave firms less competitive.
"Until such times, we can't make the key decisions which will potentially effect the further growth of our business," Mr Diamond explained.
Meanwhile, generator maker Caterpillar NI, formerly FG Wilson, has told its 1,800 staff here that the business and the whole of the UK would be better off remaining part of the EU.
It said that it had told employees "it believes the UK's continued membership of a reformed and competitive EU is fundamental to the economic growth and wellbeing of both the UK and EU economy and best serves Caterpillar's business interests".
The Remain campaign is arguing a range of reasons for the UK, including Northern Ireland, staying within the EU. That includes everything from the economic uncertainty surrounding the run-up to the vote, and what might happen afterwards, to the strength they believe firms here have within the single market to worker and human rights.