Tony Blair and Sir John Major have united to warn that Brexit could "tear apart the UK".
The former prime ministers and political adversaries shared a platform in Northern Ireland to warn that leaving the EU would be a "historic mistake" and mean "throwing all of the pieces of the constitutional jigsaw into the air."
.Speaking in Northern Ireland, the political heavyweights said a Brexit could:
undermine the peace process in Northern Ireland
trigger a second independence referendum in Scotland
damage the UK's relationship with the Republic of Ireland and lead to the reintroduction of a "hard border"
Blair and Sir John, both key figures in the Northern Ireland peace process, said a vote to leave could put the "carefully constructed foundations" of peace and stability at risk.
Their warning was echoed by former president of the United States Bill Clinton, who said he was concerned Northern Ireland's "future prosperity and peace" was in danger if the UK voted to leave.
Sir John said the "the unity of the United Kingdom itself is on the ballot paper" when Britons head to the polls for the in-out referendum on June 23.
"The most successful union in world history would be broken apart for good," he said, warning that Scots could launch another bid for independence upon a Brexit vote.
"The plain uncomfortable truth is that the unity of the United Kingdom itself is on the ballot paper in two weeks time," Sir John added.
In a scathing attack on the Leave campaign, Blair said it puts an "ideological fixation" with Brexit ahead of the damage it would cause.
"I say, don't take a punt on these people. Don't let them take risks with Northern Ireland's future. Don't let them undermine our United Kingdom."
Mr Blair added: "It is a seismic decision, it's a decision of immense importance to the whole of the country."
Theresa Villiers, the pro-Brexit Northern Ireland secretary, said claims peace in Northern Ireland could be threatened was "highly irresponsible".
"Support for the peace process in Northern Ireland is rock solid," she said.
"The vast majority of people in Northern Ireland believe their future should only ever be determined by democracy and consent and not by violence. I very much hope figures who played such an important role in the peace process would not suggest that a Brexit vote would weaken that resolve in any way.
"Whatever the result of the referendum, Northern Ireland is not going back to the troubles of its past and to suggest otherwise would be highly irresponsible."
Northern Ireland's First Minister Arlene Foster described the Remain campaign as "pretty desperate" and called Mr Major and Mr Balir "faces from the past."
Arlene Foster says Northern Irish voters will "vote as they see fit":