Democratic Unionist Party leader Peter Robinson has lost his Westminster seat to Lord Mayor of Belfast Naomi Long of the Alliance Party.
Ms Long received 12,839 votes, while Mr Robinson received 11,306 in the Belfast East constituency.
The seismic result, which represented a 22.9% swing in support, comes in the wake of a series of damaging revelations about both the DUP leader and his wife Iris.
Mr Robinson remained in his role as Northern Ireland First Minister in the wake of the scandals, insisting he had done nothing wrong.
However, the loss of a seat he has held since 1979 will cast doubt whether he can survive as First Minister and leader of the party.
In the wake of the result, which saw his 6,000 majority turned into a 1,500 vote reverse, he vowed to fight on.
'I have a job to complete with my mandate at the Assembly and I will continue to carry out that important work.'
Ms Long, whose win secured the cross-community Alliance party's first ever Westminster seat, thanked the voters for backing her.
'I went to the electorate with a track record of hard work with a passion for east Belfast where I have lived in my whole life and with a dedication to serve them in an open and transparent way and give them my best and to build a better future for everyone in east Belfast.'
She trailed in third in the 2005 poll, more than 11,000 votes behind Mr Robinson.
Reacting to the East Belfast result, Northern Ireland Deputy First Minister Martin McGuinness described it 'as a major shock' and a dramatic development.
He said he would work with Mr Robinson up until the next Assembly election.
Taoiseach Brian Cowen welcomed indications that the former East Belfast MP is willing to stay on in the post.
Speaking in Dublin this afternoon, Mr Cowen said he had been in touch to commiserate with Mr Robinson on the loss of his seat.
He said the formation of a government was exclusively a matter for the parties at Westminster and he insisted he would be willing to work with whoever emerged as Prime Minister.
Gildernew wins Fermanagh/South Tyrone
The results for all 18 constituencies in Northern Ireland have now been decided.
Sinn Féin's Michelle Gildernew won Fermanagh/South Tyrone, the final Westminster seat from Northern Ireland to be decided, after a third recount.
In the original count, Independent Unionist candidate Rodney Connor was in front by eight votes, Ms Gildernew was ten votes ahead after the first recount and that margin was cut to two after the second recount.
In the third and final recount, the NI Minister took the seat with a four-vote majority.
In total, the DUP won eight seats, Sinn Féin has five, the SDLP has three, one seat for the Alliance Party and one Independent.
DUP candidates Nigel Dodds, Sammy Wilson, Gregory Campbell, Jeffrey Donaldson, William McCrea and David Simpson all held onto their Westminster seats. Ian Paisley Jnr was elected to his father's seat and Jim Shannon topped the poll in Strangford.
Paying tribute to his father, who along with his mother Eileen joined supporters at the Ballymena count centre, Mr Paisley Jnr said he would strive to emulate the former DUP leader.
'If I can follow in those footsteps ever so carefully and leave any sort of impregnation on the sands and soils of this wonderful constituency I will try, but no man in Ulster will ever come up to the mark of Ian Paisley Snr,' he said.
Sinn Féin President Gerry Adams was the top vote-getter in Northern Ireland with 22,840 votes in the Belfast West constituency, while Mr McGuinness, Conor Murphy and Pat Doherty were all re-elected.
Alasdair McDonnell and Mark Durkan were returned for the SDLP, while Margaret Ritchie took the outgoing seat held by her party in South Down.
Independent Sylvia Hermon was elected in the North Down constituency, as the Ulster Unionist party failed to win a seat.
Separately, Sinn Féin President Gerry Adams has said there is no possibility that his party would take its seats at Westminster and get involved in negotiations to form a new UK government.