Any return of a border between Northern Ireland and the Republic after Brexit risks reviving memories of the worst days of the Troubles, Sir John Major has warned.
The former prime minister called for the immediate revocation of the Article 50 withdrawal process in order to give politicians on all sides time to work through the present "morass".
"The clock must be stopped," he said.
"It is clear we need the most precious commodity of all – time. Time for serious and profound reflection – by parliament and people.
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Speaking in Dublin, Sir John – one of the architects of the peace process – said Brexit had already injected "its very own poison" into the politics of Northern Ireland.
He said Theresa May's dependence on the support of the DUP in the Commons was hampering efforts to restore the power-sharing administration at Stormont.
In the two years since its collapse, he said relations between the unionist and nationalist communities had deteriorated, while Brexit had the potential to make the situation worse through the return of the border.
"Any border between north and south risks re-awakening memories of the worst of days – nearby graveyards bear witness to how bad were those days. No sensible person can wish to return to them," he said.
"A border would not just be a trade barrier. It would be a visible manifestation of 'us' and 'them'.
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"In the past, it not only divided communities, it divided minds; it created sides; it was the justification for conflict and murder; and its disappearance was one of the best days in the long history of Ireland."
While under the Withdrawal Agreement, the return of the border would be deferred for a while, Sir John expressed concern the hoped for technical solutions which would avoid the need for border controls may prove to be a "mirage" .
He said there was now an onus on the two main parties at Stormont – the DUP and Sinn Féin – needed to find a way to restore power-sharing.
"Everyone must realise that sectarianism has not yet gone: Northern Ireland is still far from a community that votes solely on policy and not identity. History's legacy is not yet forgotten – or put aside. No-one should forget that," he said.
"But – let me be blunt – if the DUP and Sinn Féin let old wounds, old suspicions, old enmities take fresh root and live again then the people of Northern Ireland will be the first to lose.
"They may not be alone: subsequent losers may include the culpable politicians. Both parties are elected to take up their responsibilities, and every day they find excuses not to do so is a dereliction of their duty to their electors."
He stressed the need for the UK and Irish governments to establish permanent bodies which guaranteed continued regular meetings between their respective premiers and other senior ministers and officials after Brexit.
"Inside or outside the European Union, we will always have much to discuss with our nearest neighbour," he said.
"This is not just about trade. The relationship between our two countries can be good or bad; close or distant; but whatever it is will have an effect on community relations in Northern Ireland."