BREXIT could cause "profound damage" to the north's peace process, a former Secretary of State has said.
Lord Hain, a Secretary of State under Tony Blair, warned that a return to checkpoints along the border would sow division and discontent.
The British government must not trigger Article 50 and start formal talks on leaving the EU before enshrining in law its commitment to maintaining the open border between Northern Ireland and the Republic, he said.
Speaking in the House of Lords yesterday, as he proposed an amendment to the European Union (Notification of Withdrawal) Bill which would include such a commitment, he warned against the "grim peril" of a hard Brexit.
"The settlement in Northern Ireland is built on a delicate balance of the three strands of the Good Friday Agreement - relationships within Northern Ireland, between Belfast and Dublin, and Dublin and London," he said.
"Brexit will test each of these relationships, and if the government pursues a hard Brexit it could do profound damage to all three."
The Labour peer added: "Frankly I'm not convinced the government has even begun to grasp the political significance of it."
Labour peer Lord Murphy, a former Secretary of State, also said the question of the border should be "top of the agenda".
Former Ulster Unionist MP Lord Kilclooney, said the "southern Irish are petrified at the impact of Brexit" as "thousands and thousands" come north every day to shop.
Lord Hain later withdrew the amendment following reassurances from Lord Dunlop, Parliamentary Under Secretary of State for Northern Ireland, who said the government is absolutely committed to maintaining and building upon the peace process.
The debate came as it was reported that British Prime Minister Theresa May is set to announce the end of free movement for new EU migrants next month.
The Daily Telegraph said she is expected to say that European citizens who travel to the UK after Article 50 is triggered will no longer have the automatic right to stay permanently. They are also expected to be subject to a new visa system.
The report led to speculation that as well as residents of the Republic, the change may affect people in the north who hold Irish but not British passports.
