“It’s unclear what the border arrangements would be if Britain were to leave the European Union,” Mandelson said in an interview on Newstalk on Tuesday morning. “You’d want to avoid border posts, you’d want to avoid elaborate checkpoints - but I fear that they would be inevitable because this would be the only land border that Britain has with another country,” he said.

The former British Labour Party cabinet minister will be addressing the British-Irish chamber of commerce in London on Tuesday evening in relation to the matter.

Because it would be the only border separating Britain from the European Union, it would have to be policed in some way, Mandelson says. This would be a setback for Ireland, he says, both north and south.

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“It certainly wouldn’t be business as usual, and I think the re-introduction of such a hard border would be a setback for all of Ireland.

I couldn’t imagine that this would help trade, business, the movement of people, the progress seen over the years, the peace in Northern Ireland, and the emergence of a strong – and potentially very strong – Irish economy

“I think it would be a threat to the North’s progress, it would jeopardise trade and jobs, and it would complicate the relationships that underpins the peace across Ireland. I think it’s the last thing people in either the north or the south would want to see,” he said.

Special arrangement

Britain takes to the polls on 23 June to decide whether or not to remain in the EU. If Brexit happens, could there be a special arrangement for Ireland in terms of border control?

“It could be the case – but it might not be the case. Britain would be leaving the single market, with separate laws and regulations. We would have to police for trade and other purposes to protect UK territory, and that would be in the Republic of Ireland, inevitably.

“I couldn’t imagine that this would help trade, business, the movement of people, the progress seen over the years, the peace in Northern Ireland, and the emergence of a strong – and potentially very strong – Irish economy,” he said.

In terms of foreign direct investment and corporate tax come, Mandelson says he couldn’t see Northern Ireland being separate from the UK for tax purposes, and Northern Ireland’s GDP could be hit by as much as 3%.

“If we destabilise Northern Ireland economically it would have political consequences. We would be destabilising the peace process, all for the sake of the political aims of people who can’t stand the idea of Britain cooperating with its European partners in the EU.

“It also creates a fresh threat of Scotland’s detachment from the UK. It will almost certainly spark a fresh referendum with inevitable results,” he said.

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