Former Northern Ireland Secretary Peter Mandelson has warned a UK exit from the European Union could result in "renewed sectarianism", risking jobs and peace.
Lord Mandelson said the EU had been central to the peace process and "an enabler of peace in Northern Ireland".
He claimed a UK exit would lead to new Irish border controls, setting back peace process achievements, and putting 50,000 Northern Ireland jobs at risk.
But the Democratic Unionist Party (DUP) dismissed his claims as "nonsense".
DUP MP Sammy Wilson described the peer as the "master of spin" and said his predictions about the consequences of leaving the EU "should be treated with the strongest degree of scepticism".
Lord Mandelson was secretary of state for Northern Ireland from October 1999 until January 2001 and is also a former European commissioner for trade.
He is campaigning for the UK to remain in the EU in the forthcoming referendum in June.
Addressing a London conference on the issue, the peer said: "Anything, in my view, that strengthened a sense of separation between Northern and southern Ireland - physically, economically, psychologically - has the potential to upset the progress that has been made and serve as a potential source of renewed sectarianism."
Earlier, he told BBC Radio Ulster, a British exit (Brexit) would mean "almost inevitable change" to Irish border controls as it would be the only land border separating the UK from the EU.
He warned that reinforcing separation between Northern Ireland and the Republic of Ireland would "only serve the extremists and those who foment the sectarianism that breeds violence and we don't want to go back to those days".
He also said EU membership had helped to boost the economy, which was "the greatest platform for sustainable peace".
He told the BBC: "There are 50,000 Northern Irish jobs linked to exports to the European Union and if we start to insert new trade as a result of Britain leaving the European Union, which will limit demand for those exports, these jobs would be put at risk."
However, Mr Wilson said the peer's comments on the consequences of leaving the EU were "not just spin, they would be a credit to the most energetic acrobat".
"His claim that 50,000 jobs would be at risk is just plain nonsense. For any such prediction to be true, every single good sold to the EU countries including the UK would have to stop being traded," the DUP MP said.
"As for his claim that we would have a hard border with border controls etc he knows full well that the common travel area between the UK and the Republic of Ireland is not dependent on EU membership.
"Indeed, it is as important to the Republic of Ireland as it is to us and the open border at present does not cause problems of large scale illegal immigration at present."