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Sir John Major and Tony Blair will share a platform for the first time to warn that leaving the EU is a threat to peace in Northern Ireland.

The former arch-rivals, who carved out the peace process in successive spells as Prime Minister, will say the fragile deal must not be put at risk.

Leave campaigners have failed to explain how Northern Ireland could maintain its open borders with the Republic if Britain revives a full border and work visa system with the EU.

Tory Sir John will tell young people in Northern Ireland any move that destabilised peace would be “a dreadful mistake”.

(Image: Getty)

He adds: “But that is what a British exit from the EU would do. It would throw all of the pieces of the ­constitutional jigsaw into the air again and no one could say where they might land.”

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Alongside him, Labour’s Mr Blair will warn that Northern Ireland’s stability and prosperity is “poised on carefully constructed foundations”.

He adds: “And so we are naturally concerned at the prospect of anything that could put those foundations at risk.”

Mr Blair will say the common travel area between Britain and Ireland, in place for almost a century, will be in doubt if Britain erects new border controls. He adds: “If we vote to remain in the EU, the common travel area stays – and Northern Ireland can look forward to a bright and prosperous future.

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“But if we vote [Leave] it would put Northern Ireland’s future at risk. It would be deeply damaging and reckless.”

He said people needed answers from Leave “and they need to have them in the next 14 days”.