The Ulster Unionist leader, David Trimble, has lost his seat in Upper Bann.
David Trimble: Where now for UUP leader?
In a sensational victory DUP rival David Simpson topped the poll with 16,679 votes to Mr Trimble's 11,281.
With its leader unseated as an MP and four UUP seats now gone, the result has thrown the future of the party and Mr Trimble into disarray.
Earlier a close Trimble supporter, David McNarry, said his party had to prepare for its leader's resignation.
Mr McNarry was speaking as the explosive situation unfolded at the Upper Bann count.
He told BBC Radio Ulster: "I would expect that certainly 48 hours David will have made his decision"
"I expect David Trimble regrettably to resign his position as party leader and then the party will set about dealing with all of that."
The former Deputy leader of the UUP, Lord Kilclooney, told the BBC that he would resign if he was in Mr Trimble's position.
Speaking a short time after the result was announced Mr Trimble said his record over the last number of years would be there.
"I believe the situation Northern Ireland is now in a much better one as a result of what we have done.
"I am proud of our record.
"Today has been a considerable success for the Democratic Unionist Party. But, they will know that with that success comes responsibility.
"They have inherited from Ulster Unionism a very strong position for unionism and I hope they manage to safeguard that position over the course of the months to come."
David Simpson said of his own victory: "It sends out a very, very clear signal that push-over unionism has gone forever."
The DUP deputy leader, Peter Robinson, said of Mr Trimble: "Slowly and surely he has been brought to account.
"You can't play fast and loose with the electorate."
During the election campaign Mr Trimble astonished observers by insisting his party would not lose any seats despite indications to the contrary.
For their part the DUP pointedly held their election manifesto launch in Upper Bann in an indication of how much they prized this seat.
Although he was not one of the DUP's leading lights Mr Simpson has in recent times had a strong local profile.
Not only is he Mayor of Craigavon, he is also a successful businessman who frequently appears in the local press.