Irish Prime Minister Enda Kenny said on Monday a Brexit would create instability and uncertainty, as well as the likely return of border checkpoints between Britain and the Republic of Ireland.
"Stability and certainty are absolutely essential to successful small open economies," Kenny said in a speech at Ulster University, Belfast, referring to how this would affect the relationship between Northern Ireland -- a part of Britain -- and the Repbulic of Ireland.
Continued British membership of the European Union (EU) offers stability and certainty, he said.
Kenny said tens of thousands of people cross the border between the each day to work, do business, and see family and friends.
"It is hard to imagine life without this free flow," he said.
"The reestablishment of customs checks on the border, or indeed of any customs arrangements, would be a regrettable and backward step for north-south trade and cooperation," he added.
Kenny stressed his government's strong support for a vote to remain in the EU.
Britain will hold a referendum on June 23 to decide whether or not it should stay in the EU.
"That decision is as important for the future of this island as when we all voted for the Good Friday Agreement," Kenny said.
The Good Friday Agreement or Belfast Agreement, signed on April 10, 1998, was a major political development in the Northern Ireland peace process of the 1990s.
The Irish prime minister said there was no doubt that leaving the EU would involve changes to the trading rules between Britain and Ireland.
"Can anyone credibly suggest that nothing would change if that became the western border of the European Union?" he asked.