Dr David Shiels is a contemporary historian and College Research Associate at Wolfson College, Cambridge.

Will the British Union survive if the UK votes to leave the European Union in June? This question is frequently posed in relation to Scotland, but rarely addressed in relation to Northern Ireland. For Unionists in Ulster, however, it is a question of supreme importance and one that raises questions about the long-term survival of Ulster Unionism as a political ideology. As expected, the issue has divided the two main Unionist parties as they seek to come to terms with what a British withdrawal from the EU might mean for the long-term future of Northern Ireland.

From a Unionist perspective, there are strong grounds for being a Eurosceptic or even an Outer. The relationship between Ulster Unionism and Euroscepticism goes back to the original debates on British membership of the EEC in the 1960s and 1970s. In the 1975 referendum, Ian Paisley warned that a vote for the Common Market “is a vote for Ecumenism, Rome, Dictatorship, Anti-Christ”. The Ulster Unionist party had Enoch Powell, the founding father of modern Tory Euroscepticism, who had left the Conservatives in protest at the party’s stance on Europe at the February 1974 General Election.

In more recent years, the DUP have remained true to their Eurosceptic roots, albeit using more moderate language, while the Ulster Unionists have accepted EU membership on pragmatic grounds. Indeed, the DUP contains some of the most enthusiastic supporters of the “Vote Leave” campaign. Even before the results of the renegotiation were announced, several DUP MPs had declared their intention to campaign for Brexit come what may. The party’s new leader, Arlene Foster, is a little more cautious, advocating a Leave vote “on balance” – and there is a sense that the DUP Ministers in the Stormont Executive would be relieved if they did not have to deal with the consequences of Brexit. All the same, the party is now strongly identified with the Vote Leave campaign, and would be in a good position to work with a post-Cameron leadership if they were on the winning side of the referendum campaign.

On the other side of the debate, the Ulster Unionist party has argued that the UK is “on balance” (that phrase again) better off inside the EU. The UUP leader, Mike Nesbitt (who stood as a Conservative and Unionist candidate at the 2010 General Election), met David Cameron at 10 Downing Street before putting the matter to a vote at the party’s executive. He seems genuinely committed to the view that membership of the EU is vital to Northern Ireland’s continued prosperity and claims he does not want his fingerprints on the break-up of the UK by enabling the SNP to call a second referendum on Scottish independence. At the same time, there are a number of individuals within the party – notably the former MEP and deputy leader, Lord Kilclooney – who have said they support Brexit.

The debate in Northern Ireland is, of course, seen through the prism of the peace process and the future of Anglo-Irish relations. Many Unionists are uncomfortable with the political aspects of the European project, feeling that movement towards political unity at a European level undermines the British union. The EU has been a better and more effective agent of ‘harmonisation’ between Northern Ireland and the Republic of Ireland than any North-South body set up by the British and Irish Governments. It is probably only in the context of the Republic’s EU membership that Irish unity would ever be a viable option for either state.

For these reasons. most Unionists have an ingrained hostility to the EU, and they are inevitably suspicious of the way that the Irish Government has been speaking out against Brexit. When Enda Kenny recently claimed that Brexit would create a “serious difficulty” for Northern Ireland, the DUP’s leader at Westminster, Nigel Dodds, warned that it was “disrespectful” and “counterproductive” for the Irish Government to intervene in the internal affairs of Northern Ireland. Similarly, Lord Trimble, the former First Minister, now a Tory peer and a supporter of Brexit, has said that there is “no connection between the peace process and the European Union…I think this is just scaremongering”.

On the other hand, the concerns of the Irish Government should not be dismissed completely. There is no doubt that Brexit would open up lots of difficult questions for the UK and Ireland, involving the border, citizenship rights and mutual trade. We have heard very little from the Leave side – including the Northern Ireland Secretary, Theresa Villiers – about the impact that Brexit would have on UK-Ireland relations.

Added to all of this is the position of the Nationalist politicians in Northern Ireland, who see membership of the EU as vital to their goal of pursuing a united Ireland. Even Sinn Fein has said that it will “campaign vigorously” in support of the EU – the first time that the party has ever advocated a pro-EU vote in Northern Ireland or the Republic. Like the SNP in Scotland, Sinn Fein has moved away from its traditional Eurosceptic position in favour of a more calculated policy of supporting the EU as a vehicle for Irish unity. They will, however, do their best to antagonise Unionists in the meantime.

Just as Nicola Sturgeon has said that Brexit would “almost certainly” trigger a second independence referendum in Scotland, Martin McGuinness has raised the possibility of a border poll in Northern Ireland in the immediate aftermath of British withdrawal from the EU. If nothing else, this shows that the Sinn Fein leadership knows how to exploit Unionist divisions.

What, therefore, are grassroots Unionists to make of Brexit? While some will follow existing party loyalties, it is clear that many Unionists are genuinely torn on the issue of how to vote in the referendum. At the moment, the case for Irish unity seems as fanciful as the case for Scottish independence, but there is always a danger that the separatist parties could exploit a political and constitutional crisis caused by Brexit for their own advantage.