Prime Minister David Cameron has warned of checks along the Northern Ireland border with the Republic and even the possibility people travelling from Belfast to other parts of the UK will have to provide paperwork in the event of a Brexit in the EU referendum.
Mr Cameron was responding to a question from former SDLP leader, the MP Alasdair McDonnell.
Dr McDonnell raised fears that there could be a return to hard borders and passport checks between Northern Ireland and the Republic.
He said: "The Brexit campaigners have made securing our borders their resounding war cry, but when it comes to the only land border between the UK and the rest of the EU we’re dismissed and told that nothing will change there.
"A return to custom posts, passport checks and a hard border will be a critical economic issue for Northern Ireland’s voters in eight days’ time.
"So can the Prime Minister now, once and for all, clarify this issue and say what will become of the border if the UK votes to leave the European Union?”
David Cameron responded: “If we vote to stay in we know what the situation is.
"We know that the Common Travel Area works, we know it can continue and everyone can have confidence in that.
"If we were to Leave, and, as the Leave campaigners want, make a big issue about our borders, then you’ve got a land border with Britain outside the European Union and the Republic of Ireland inside the EU.
"Therefore you can only either have new border controls between the Republic of Ireland and Northern Ireland, or, which I would regret hugely, you would have to have some sort of checks on people as they left Belfast or other parts of Northern Ireland to come to the rest of UK.
"Now we can avoid these risks, there are so many risks here, risks to our children’s job, risks to our borders, risks to the unity of the United Kingdom – I say avoid the risks and vote Remain next Thursday.”