Brexit could cause “profound damage” to the Northern Ireland peace process, a former Cabinet minister has said.
Lord Hain, the former Northern Ireland Secretary under Tony Blair, warned that a return to checkpoints along the border would sew division and discontent in the region.
The 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 of Ireland, he said.
Speaking in the House of Lords as he proposed an amendment to the European Union (Notification of Withdrawal) Bill, which would enshrine such a commitment, he warned against the “grim peril” of a hard Brexit.
Lord Hain said: “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.
“Brexit will test each of these relationships, and if the Government pursues a hard Brexit it could do profound damage to all three.”
Any physical border will create ill-feeling and could destabilise the region and “unravel” the peace process, Lord Hain warned.
Yet this danger is not addressed in the Government’s white paper on leaving the EU, the Labour peer said.
He added: “Frankly I’m not convinced the Government has even begun to grasp the political significance of it.”
Lord Hain said the open border is a symbol of normalisation between the two parts of the island, adding: “The Government stirs that at everyone’s great and grim peril.”
He said: “If the referendum means Brexit at any price, it may well be at a dangerously high cost for the Northern Ireland peace process.”
He urged ministers to back his amendment, stressing that it only seeks to lay down in law what Theresa May has already committed to verbally.
Lord Hain said: “Ours is not a wrecking amendment. It does not obstruct Brexit, it does not tie the Government’s negotiating hand.
“All it is dong is insisting that, as Article 50 is triggered, it is only on the basis that the Government negotiates to secure what it already says it wants - an open border in line with the Good Friday Agreement.
“I trust, my lords, that we never have to confront the stark choice between delivering on the Brexit referendum and deepening hard-won stability and peace on the island of Ireland.”
Liberal Democrat Lord Alderdice said it was in the interests of Brussels to be flexible over maintaining an open border.
Former Ulster Unionist leader, now Tory peer Lord Trimble said the amendment was unnecessary.
It asked the Prime Minister to support the maintenance of an open border but she did that “as of now” and had included it in the Government’s White Paper on Brexit.
The Government’s objective was tariff-free access, he said. If it achieved that there was no problem but if tariffs were introduced this would have “serious implications” for the Republic.
Former government spokeswoman on Northern Ireland and Lib Dem peer Baroness Randerson said UK political credibility in Northern Ireland was at a “low ebb” and Brexit had already done “damage” in the Republic.
She said the open border was a “powerful symbol” of the peace process and a return to border posts would be “hugely damaging”.
Former head of the civil service Lord Kerslake, whose mother had come to England from Ireland after the war, said: “The question is not whether we have a border, it’s how seamless it is.
“It seems clear that the harder the Brexit, the harder the border.”
He added: “In my view our commitment to both the letter and the spirit of the Good Friday Agreement must stand above most if not all of our other ambitions.”
Baroness Harris of Richmond said: “We must have more clarity from ministers on the practical implications of Brexit for the 35,000 people estimated to cross the border every day.”
In the face of the continuing dissident threat, she also warned police officers stationed at fixed border posts “would become sitting targets”.
Tory former Cabinet minister Lord Forsyth said: “This is a Bill about starting the starting gun for Article 50.”
He added: “The very worst thing you can do in any negotiation is announce in advance how you are going to do it, because you are then committed to that position and the people on the other side will make it very difficult for you.”
Labour peer Lord Dubs said the Irish government was very concerned about the issue and argued the amendment would bolster the Prime Minister’s negotiating position.
He said: “It would strengthen her resolve and she could say that the British Parliament was so concerned about it we have put it on the face of the Bill.”
Former Ulster Unionist leader Lord Empey said he had heard no one “say that they want or wish to see a hard border”.
“Nobody wants it,” he added.
“I believe with that sort of momentum, I believe we will find means.”
He argued it was “a mistake” to link the Belfast Agreement with triggering Article 50.
“They are two totally separate things,” said the peer.
However, Lord Empey did seek an assurance from the Government there would be no internal border within the UK.
“You cannot be ambiguous on these issues.
“There’s either a United Kingdom of Great Britain and Northern Ireland or there isn’t,” he said.