Leaving the European Union would undermine the Northern Ireland peace process and increase the risk of renewed sectarian violence, Lord Mandelson has warned.
The Labour peer – who served both as a European Commissioner and Northern Ireland Secretary – said the reimposition of a “hard border” with the Republic would only bolster extremists.
The EU had been “an enabler of peace of Northern Ireland and a fundamentally stabilising presence in Ireland’s recent history”, he claimed – saying shared membership had help “underpin” the Good Friday Agreement.
“Why would we sacrifice such a steadying force for uncertainty and unknowns?” he asked in a speech to the British Irish Chamber of Commerce in the City of London.
He said: “The direction of policy for the last 30 years has been to make this border less prominent and less conspicuous for all the reasons we understand.
“For practical purposes it has all but disappeared.
“As a result there is greater political cooperation and indeed tentative steps now towards a much more prosperous and sustainable all-Ireland economy.
“It is unclear what the border arrangements would be in the unprecedented situation of the UK leaving Europe.
“Everyone would want to avoid border posts and elaborate checks but who knows what would have to be imposed, who knows what would have to be put in place on the recreation of that hard border?
“Certainly one thing is true: the reimposition of a formalised border would be a radical departure from the established strategy of the administrations in Dublin, London and Belfast.
“Anything in my view that strengthened a sense of separatism between Northern and Southern Ireland – physically, economically, psychologically – has the potential to upset the progress that has been made and serve as a potential source of renewed sectarianism that would always bear the risk of triggering further violence in Ireland, particularly in the North.”
Lord Mandelson said Northern Ireland’s GDP could drop by as much as three per cent after Brexit – with 50,000 trade-related jobs “at risk”.
“It would mean spending cuts, it would mean rising taxes, it would mean unemployment rising and it would mean investment falling.
“What we have created in Northern Ireland is something really incredible, terribly special but also fragile – and we should not be taking risks with what we have created,” he said.
“We have achieved too much and gone too far simply to throw it away.
“Our cultural, economic and familial ties would not be severed but they would be impacted.
“This not only underlines the seismic nature of this decision but the scale of the risk if we leap in the wrong way.”
Of the peace process, he said: “You start pulling threads in that and almost in unidentifiable ways, you will find the cement loosening in that wall of strength that we have created that keeps the peace process up and running.”
In comments he denied were directed at the DUP or any other group, he told the audience: “There will be people who say ‘actually the more separatism we have the better, put that hard border back in place, let’s make it more difficult to trade, more difficult to travel’.
“It would keep people’s identities apart they might think; it would keep them living apart they might say; it would keep them growing apart they might suggest.
“But in my view that sense of separatism that would emerge from the reimposition of that border would only serve the extremists, the sectarians.”
Lord Mandelson said the referendum campaign would be “ferociously fought” and warned the pro-EU side had to make more effort to win the emotional as well as the practical arguments.
“There are many people in Britain who feel they are pulled one way by their heart and in the opposite direction by their head so there is going to be a battle royal for the hearts and minds.”
He said the only world leader who backed Brexit was Russian president Vladimir Putin.
“Why? Because he isn’t that interested in European unity, he’s not actually all that interested in the European Union surviving.”