Two former prime ministers will warn of the risks Brexit poses to the unity of the UK as they campaign in Northern Ireland.
Former adversaries Sir John Major and Tony Blair will share a platform for the Remain campaign, warning that a vote to leave the European Union could damage Northern Ireland and potentially lead to the break-up of the UK.
Former Tory premier Sir John and his Labour successor Mr Blair were instrumental in the peace process in the country and will warn against a move which could put that stability at risk.
Sir John will say : “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.”
Warning that “t he unity of the United Kingdom itself is on the ballot paper in two weeks’ time”, the former prime minister will add there is a “serious risk” that Brexit could lead to a second independence referendum in Scotland, and this time the country could vote to leave the UK.
“The most successful union in world history would be broken apart for good,” he will say.
Mr Blair will 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.”
He will add: “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.”
But Brexit-backing Northern Ireland Secretary Theresa Villiers 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.”
She insisted the common travel area between the UK and the Republic of Ireland would continue if there was a Leave win, even though the border would become the frontier between an EU member and a non-EU nation.
Ms Villiers said: “There would be risks to manage but they are not significantly more serious than risks that are already managed effectively today through bilateral co-operation between the UK and Ireland.
“The idea that thousands of non-Irish EU citizens would suddenly start crossing the border is far-fetched.”
Meanwhile, the rival camps clashed over the NHS as Tory MP and Commons Health Select Committee chairwoman Sarah Wollaston switched her allegiance from Leave to Remain.
She said she changed sides because she did not believe the key anti-EU argument that withdrawal would hand the NHS an extra £350 million a week.
In a post on her blog she said: “The claims about health from the Leave campaign have been shameful. They have knowingly placed a financial lie at the heart of their campaign, even emblazoning it on their battle bus alongside the NHS branding to imply a financial bonanza. It’s an empty promise and one which would soon backfire.”
Brexit-backing Tory John Redwood said he hoped Dr Wollaston would reconsider, but acknowledged the £350 million was the gross figure of the UK’s contribution to the EU, before the rebate and the money that came back to the UK.
Former cabinet minister Mr Redwood told BBC Radio 4’s Today programme: “O ur Brexit budget has always concentrated on all the money we do send to them that we don’t get back, which is about half of that gross total.”
He added: “I hope Sarah will think again because she, like me, thinks we need to spend more money on health.
“We can do so out of all the money that we save and we would also be able to give that cancellation of VAT on fuel to people’s households.”
MPs will vote later on emergency legislation allowing a 48-hour extension to the deadline to register for the landmark poll after the official website crashed, leaving tens of thousands of would-be voters in limbo on Tuesday.
Downing Street insisted the action was legal, despite some in the Leave camp claiming it was an attempt to ensure more Remain supporters got on the voting rolls in time for the June 23 showdown.
Leave.EU chief Arron Banks indicated he could seek a judicial review of the decision.