The British Chancellor has warned a Brexit would mean implementing new borders between the Republic of Ireland and the North.
George Osborne is campaigning in the North today in advance of the Brexit vote in three weeks.
He said a vote to leave the European Union would cause an economic shock in the North, forcing the return of border check points, slashing farmers' income and costing the region £1.3bn.
"Let's be clear, if we quit the EU then this is going to be the border with the European Union," he said, pointing to the Carlingford Lough waterway which separates Ireland North and South.
"And all the things that those that want to quit the EU claim would happen - ie new immigration checkpoints, border controls and an end to free movement - that has a real consequence, and there would have to be a real hardening of the border imposed either by the British government or indeed by the Irish government."
Leave campaigners in Northern Ireland, including Secretary of State Theresa Villiers, have insisted the impact on the Irish border will not be significant in the event of an exit.
Mr Osborne painted a very different picture of the border post-Brexit as he met with port workers in Warrenpoint.
He said the imposition of trade tariffs to export goods into the EU would rock Northern Irish businesses trading with the Republic.
"I was just talking to a guy who drives a truck - he remembers when it used to take two hours to get across the border and, as a result, business wouldn't come here, jobs wouldn't come here, people would trade directly with the Republic," Mr Osborne said.
He also pointed to a new analysis of Treasury figures to suggest unemployment would rise by 14,000 in Northern Ireland over two years if the UK left the European Union, with 2,000 added to the youth unemployment figure.
He said the impact of the shock from leaving the EU and the free trade single market could equate to a £1.3 billion reduction in the size of the Northern Ireland economy by 2018, with house prices falling by £18,000 over the same period.
"This would become the border with the European Union if we vote to quit, and that would mean fewer jobs in Northern Ireland, that would mean family incomes hit, it would mean the value of people's homes and pensions would fall, and that's not a price worth paying," he argued.
"Why take this leap in the dark with all the costs and risks associated with it? There is a strong, brighter future in a reformed EU if we vote to Remain."
Mr Osborne also stressed the need to maintain strong links with the Republic in the context of co-operating against security threats.
He added: "When it comes to Northern Ireland and its future, I would say we have made massive progress over the last 20 years, massive progress in my lifetime, and one of the things you have seen is essentially the disappearance of the border as a physical place, a check, and talking with the folk here, they were saying people who work here live on both sides of the border, people travel easily.
"So you just don't know what would happen as the border starts to harden and this becomes the external border with the European Union, and here we would be on the wrong side of it."
Mr Osborne said Brexit would spell particular bad news for Northern Ireland's farmers, who currently receive hundreds of millions of pounds of direct annual support through the EU's Common Agricultural Policy (CAP).
"Farmers get two benefits from the EU," he said.
"Firstly they can trade their products and goods - their milk, their beef, their lamb and the like - and one of the things the EU does is have very tough external tariffs on trading into the EU, and our farmers would suddenly be outside the EU paying a tax to send things to the Republic and continental Europe.
"And then you have the Common Agricultural Policy - and that's direct support to farmers. Close to 90% of farming incomes here come from EU support payments.
"What would happen if we left? The country, the whole UK, would have less money.
"Why would we have less money? Although we wouldn't be paying into the EU budget, our economy would be poorer, we 'd be raising less taxes, there would be less money for public services, so a Chancellor of the Exchequer would have to make choices - the budget would be shrinking and where are the cuts going to be felt? And I can't see why farming would be excluded from those cuts."
One of the key messages of Mr Osborne's visit was a call to voters who have not already registered to do so before Tuesday's deadline.