TWO former prime ministers - Sir John Major and Tony Blair - have been labelled "desperate men" as they insisted a Brexit could jeopardise the future of the United Kingdom.

On Northern Irish soil - in a trip loaded with history - the ex Tory and Labour premiers are arguing today a vote to leave the UK threatens the fabric of the country - including the future of reconciliation in Northern Ireland and Scotland's position in the UK.

8 Sir John Major and Tony Blair have teamed up to say they believe Brexit will damage peace in Northern Ireland Credit: PA

Sir John said: "I believe it would be a dreadful mistake to do anything that has any risk of destabilising the complicated and multi-layered constitutional settlement that underpins stability in Northern Ireland.

"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.

"So throw away membership of the European Union and don't be surprised if we accidentally throw away our union as well.

8 Sir John Major and Tony Blair say people should make a 'positive and patriotic choice' to remain Credit: PA

8 Former prime ministers Tony Blair and Sir John Major say it's essential for Britain to vote to stay in the EU Credit: PA

8 Sir John Major and Tony Blair chat as they walk across the Peace Bridge in Derry, Northern Ireland, before their pro-EU speech Credit: Getty Images

Sir John Major and Tony Blair speak as they walk across the Peace Bridge in Derry, Northern Ireland, before their pro-EU speech

"My plea is this: don't put that at risk. Make a positive and patriotic choice to remain.

"Choose to fight for the UK's interests in Europe and in so doing safeguard the UK's interests at home as well."

And Mr Blair has hit out at the Leave campaign, claiming 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."

8 Tony Blair and John Major pictured during a q&a session with children in Belfast in 1998 after working together on the Northern Ireland's Good Friday Agreement Credit: Reuters

8 John Major and Tony Blair are reunited again today after working on the Good Friday Agreement in 1998 Credit: PA

Mr Blair added: "We understand that, although today Northern Ireland is more stable and more prosperous than ever, that stability is poised on carefully constructed foundations.

"And so we are naturally concerned at the prospect of anything that could put those foundations at risk."

The two architects of the peace process insist the Leave campaign will find it impossible to restrict immigration from Europe yet maintain free travel between the Republic of Ireland and the UK.

And they say ministers would face a choice between allowing a back-door route for illegal immigrants through Northern Ireland, or abandoning the Common Travel Area and imposing border controls.

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But the pro-Brexit DUP immediately hit back setting up a bitter war of words.

MP Ian Paisley Jr described them as "two desperate men prepared to say any desperate thing to support a desperate cause".

The anti-EU MP insisted: "Given the reputation they have in Northern Ireland they will not win any support amongst the unionist community and will be desperate to hang onto any embarrassed supporters amongst the nationalist community."

8 Ian Paisley Jr has hit out at the two former prime ministers for their comments about Northern Ireland Credit: Pacemaker Press

8 Theresa Villiers said most people in Northern Ireland believe their future should be determined by democracy Credit: Reuters

And Cabinet minister Theresa Villiers insists Northern Ireland could flourish outside the EU and the open border could be kept.

She said: "Support for the peace process in Northern Ireland 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.

“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."