A group of current and former senior Ulster Unionists have appealed to party members to vote Leave in next week's EU referendum.
It includes the former party leader David Trimble and some former MPs.
The party's referendum position is that it believes, on balance, Northern Ireland is better remaining in the EU, although it respects that some of its members may vote for a withdrawal.
But some party grandees have written a letter urging members to vote Leave.
They say Ulster unionism has always been about "protecting the sovereignty and independence of the entire UK" and the party should not abandon that position.
Voters will decide on the future of the UK's EU membership on Thursday.
Among the signatories of the letter are Lord Trimble, Lord Kilclooney and Lord Maginnis, who have all left the party, as well as their former Westminster colleagues Roy Beggs and David Burnside.
In their letter, they reject what they call the "dishonest scaremongering" that a vote to leave will undermine peace in Northern Ireland.
That claim has been made by Tony Blair and Sir John Major, among others.
The former prime ministers visited Londonderry earlier this month and said the "unity of the UK itself is on the ballot paper" in next week's poll.
Mr Blair added that Northern Ireland's prosperity and political arrangements could be negatively affected by a vote to leave.
Lord Trimble, the former Northern Ireland first minister, led the Ulster Unionists through peace talks that paved the way to the Good Friday Agreement in 1998 and the creation of a power-sharing executive at Stormont.
He and the others behind the letter say the agreement provided "robust structures to enable ministers north and south to reach agreed positions in the interests of the island as a whole".
"We have no doubt that these will prove invaluable in reaching sensible arrangements with the Republic of Ireland following a Leave vote," they add.
They claim a UK withdrawal from the EU "has the potential to kick-start a British renaissance of global proportions".