Home Secretary Theresa May has said it is "inconceivable" that there will not be any changes on border arrangements with the Republic of Ireland, if the UK pulls out of the European Union.
She was speaking on a visit to Northern Ireland to campaign for a remain vote in Thursday's EU referendum.
She acknowledged that the British-Irish Common Travel Area pre-dated the EU.
But she said that if the UK pulled out of the EU, this would lead to tariffs requiring some form of controls.
Mrs May said peace in Northern Ireland is important and it would continue, whatever the result of Thursday's referendum.
However, she said there were good security reasons for remaining inside the EU, such as the access to European arrest warrants.
She had previously argued that the UK should withdraw from the European Convention on Human Rights, which is not connected to the EU but a treaty linked to the separate Council of Europe.
Mrs May stood by her comments but made the point that the Human Rights Convention is not what people are being called on to vote about on Thursday.