Updated - Former UK Prime Ministers Sir John Major and Tony Blair have warned that a vote to leave the EU would be a "historic mistake" which could threaten Northern Ireland's hard-fought peace process.
The former adversaries shared a platform at the Ulster University's Magee campus in Londonderry, and warned that Brexit could lead to the break-up of the UK.
Sir John said: "I believe it would be an historic mistake to do anything that has any risk of destabilising the complicated and multi-layered constitutional settlement that underpins stability in Northern Ireland."
Both men played crucial roles in the Northern Ireland peace process, and Sir John warned that the "wrong outcome on June 23 could "tear apart the UK".
He added: "If we throw the pieces of the constitutional jigsaw up in to the air, no one can be certain where they might land."
Mr Blair hit out at the Leave campaign, claiming it puts an "ideological fixation" with Brexit ahead of the damage it would cause.
He said: "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.
"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."
He also spoke of the potential damage to British-Irish relations if the UK withdrew from Europe.
Former US president Bill Clinton, whose 1995 visit to Northern Ireland was seen as a crucial moment in the peace process, said he was worried about the potential impact of Brexit on the province.
Writing in the New Statesman magazine, Mr Clinton said: "I was honoured to support the peace process in Northern Ireland. It has benefited from the UK's membership in the European Union, and I worry that the future prosperity and peace of Northern Ireland could be jeopardised if Britain withdraws."
The former president, who worked with Sir John and Mr Blair on the peace process, added: "Transatlantic co-operation is essential, and that co-operation is strongest when Europe is united ... I hope you will stay."