A border poll should be held in Ireland if the UK leaves the European Union, the deputy first minister has said.
A referendum on whether the UK should stay in the EU will be held on 23 June.
Martin McGuinness said a vote to end the UK's EU membership would have "profound consequences" and would be a "political and economic game-changer".
The Sinn Féin MLA said if that happened his party would put pressure on Northern Ireland Secretary Theresa Villiers to call a border poll.
Ms Villiers has the power, under the terms of Good Friday Agreement, to call a poll on whether Northern Ireland should remain part of the UK or unite with the Republic of Ireland.
A border poll was last held in Northern Ireland in 1973 and was largely boycotted by nationalists.
Mr McGuinness said: "I have proposed to Theresa Villiers that, given the enormous significance of these issues, the British government now give a firm commitment to an immediate border poll in the event Britain votes to leave the European Union."
He added that a border poll would be a "legitimate test of political opinion that would threaten no-one".
But the Northern Ireland Office said the government had set out its position on a border poll ahead of the 2015 Westminster election.
Last year's Conservative Party manifesto said "all tests of opinion" showed that "a substantial majority" of people in Northern Ireland "continues to support the union" with Great Britain.
It added that "the circumstances requiring a border poll are not currently satisfied" and that holding a vote over the Irish border would be "costly, divisive and a distraction".
Ms Villiers has said there are "risks on both sides of this debate" but the "safer option" was to leave.
She claimed last month that Northern Ireland would remain in a strong position financially if voters opted for a withdrawal from the EU.
Her stance has been supported by the Democratic Unionist Party.
But Northern Ireland's other main parties - the Ulster Unionists, the Social Democratic and Labour Party and the Alliance Party - are backing a vote for the UK to remain an EU member.