SINN Féin and the SDLP have warned of the consequences of a hard border with just days left before a vote could force Northern Ireland out of the EU.
Party president Gerry Adams said the creation of an EU border between the two parts of Ireland would add to the problems of partition.
"The possibility that a part of the island of Ireland could end up outside the European Union while the other part stays in is not a situation that will benefit the people of this island," he said yesterday.
"Partition has created enough problems. We need to avoid adding to this.
"The Good Friday Agreement has brought about significant changes, including ensuring that the border is now all but invisible.
"People who live on either side of it travel back and forward to work, to shop, to visit relatives, often on a daily basis. Brexit threatens that important change."
SDLP South Down MP, Margaret Ritchie said border custom queues would be a "disaster."
"It cannot be over-stated how common EU regulations on safety and welfare standards have helped open up the border between the North and the South for trade and business.
"That’s why, if the UK tore up the community’s various minimum standards, customs checks along the border would become unavoidable and we would return to the days of queueing for hours to declare butter to the Gardaí.
"This would be a disaster for the 30,000 commuters who cross the border every day," the former SDLP leader said.
The UK will vote on Thursday whether to Remain in or Leave the EU. The latest polls show the two side neck and neck, however Northern Ireland and Scotland are expected to strongly back Remain.
A vote to exit the European Union would leave the 310 miles frontier between Derry and Newry as an external border of the EU with the Republic inside and Northern Ireland outside.
Sinead McLaughlin, chief executive of the Londonderry Chamber of Commerce, said businesses are "extremely frightened" at the prospect of new obstacles to trade and free movement being put in force.
"The impact of a Brexit will have a devastating economic impact on this region ... we are a frontier city, and our prospects depend on that union between the regions working effectively together.
"Instead of any barriers we need closer alliances with our neighbours."
A whole range of commentators have said customs posts are inevitable, however, Secretary of State Theresa Villiers, a Leave campaigner, claims the border will remain unchanged.
