Ulster Unionist leader Mr David Trimble fired a shot across the bows of the hardline Democratic Unionists, claiming their election strategy could hand seats to Sinn Fein.
The UUP leader and Northern Ireland First Minister warned last night that the "opportunist" DUP and other anti-Agreement factions risked splitting the unionist vote in marginal constituencies.
Last year the DUP, which bitterly opposes the new political arrangements in Northern Ireland, wrested a Westminster seat from Mr Trimble's party, winning the South Antrim by-election caused by the death of UUP MP Clifford Forsythe.
But last night Mr Trimble said the DUP's plans to field candidates in all Ulster Unionist seats in the general election would open the door to republicans.
He said at the UUP East Antrim constituency association annual general meeting: "In the forthcoming general election it is crucial the Ulster Unionist Party secures the largest vote possible. The DUP claim they will fight the Ulster Unionist Party in every constituency. Yet in the next breath they claim to stand for unionist unity.
"Any unionist who thinks that by voting in support of opportunist DUP candidates in areas such as North Belfast, West Tyrone, Fermanagh South Tyrone, should realise they will probably end up electing Sinn Fein candidates."
He was convinced the "vast majority" of unionist voters wanted the Agreement to work and he said: "There are very few pieces of this jigsaw left to put in place.
PA