THE UUP has officially declared its support for Britain remaining in the EU - leaving the DUP as the only one of the five main parties to campaign for exit.
However, the UUP will not force party members to take its stated line in the referendum vote, as well as indicating it is pressing for "further reform" and does not back closer political union between member states.
In a statement, the party said: "The Ulster Unionist Party believes that on balance Northern Ireland is better remaining in the European Union, with the UK government pressing for further reform and a return to the founding principle of free trade, not greater political union.
"The party respects that individual members may vote for withdrawal on the 23rd of June."
The announcement came after a meeting of its ruling executive in Templepatrick, Co Antrim on Saturday.
Two weeks ago, the UUP promised supporters that it would "move quickly" to come to a decision on the matter, but was waiting until "further details" about British prime minister David Cameron's deal with other EU leaders were known.
However, there were clear indications that the party would choose to side with the `In' campaign, with just last week, UUP business spokesman Mark Cosgrove warning that leaving the EU would be "cataclysmic" for Northern Ireland companies.
With Sinn Féin, the SDLP and Alliance party also supporting the UK remaining part of the EU, the Ulster Unionist move means the debate in Northern Ireland will not divide along traditional orange/green lines.
The DUP is in favour of leaving the EU, as is the TUV and UKIP.
However, while unlike the nationalist parties and the Alliance who are all actively campaigning for their side, the Democratic Unionists are leaving it to their members to decide whether to participate in events.
East Antrim MP Sammy Wilson has already taken part in first Northern Ireland Brexit-supporting public meeting, `Grassroots Out' alongside UKIP and TUV politicians.
