Tony Blair and Sir Joh Major, two former political rivals and prime ministers, have joined forces in Northern Ireland to warn of a Leave vote at the EU referendum. The Labour and Conservative grandees spoke to the University of Ulster in Derry on 9 June, with just two weeks to go before the historic 23 June vote.

The politicians both played a major part in the Northern Ireland peace process, which led to the seminal Good Friday Agreement in 1998. Major and Blair both warned that a Brexit could jeopardise the unity of the UK, particularly in the province.

"I remember when I used to come to Northern Ireland in the 1980s and the constant fear in which people lived, the constantly daily grind of conflict and horror and sectarianism and violence that dominated and often defined people's lives at the time," Blair said.

"Bringing about peace was not easy, but it was a vast co-operative effort, an effort in which Sir John Major played such an important and essential part, but one which also involved people setting aside old positions and coming together."

He added: "When we negotiated the Good Friday Agreement it wasn't easy at a whole range of levels, but one vital part of that, which people often overlook, was that it also symbolised a new relationship between the Republic of Ireland and the UK within the EU.

"One of the things that the EU had done is that it brought about a different attitude and a different sentiment that the Republic of Ireland and the UK, whatever the enmities of our past, nonetheless we could come together in part because we were also both members of the EU."

But the Northern Ireland secretary, and Vote Leave spokeswoman, Theresa Villiers hit back at Major and Blair, warning that it would be "highly irresponsible" to suggest the peace process is "rock solid".

"The vast majority of people in Northern Ireland believe their future should only ever be determined by democracy and consent and not by violence," the top Conservative argued.

"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, like the rest of the UK, will flourish outside the EU. We can keep an open land border. The Common Travel Area between the UK and Ireland has existed for nearly 100 years since the creation of the Irish state in the 1920s. It will continue if we vote to Leave.

"The idea that thousands of non-Irish EU citizens would suddenly start crossing the border is far-fetched. If we vote Leave and change the rules on free movement for non-Irish EU citizens, then if they come to the UK across our land border without legal clearance to do so, they would not be able to work, or claim benefits, or rent a home, or open a bank account and could ultimately be deported."

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