The Alliance Party's deputy leader, Stephen Farry, beat the Democratic Unionist Party front runner, Alex Easton, to take Sylvia Hermon's seat in North Down in a significant upset.
Mr Easton had been seen as the favourite to win the seat after polling just 1,200 votes behind Ms Hermon in the UK general election of 2017.
Mr Farry topped the poll with 18,358 votes, almost 3,000 ahead of Mr Easton, on 15,390. The loss will be a blow to the DUP on a night that also saw Nigel Dodds lose his seat in Belfast North.
North Down had voted Remain in the Brexit referendum, and Ms Hermon, an Independent unionist, had often been a sole voice in the House of Commons representing those in Northern Ireland who wished to stay in the European Union. Mr Farry's electoral success lay in hoovering up Ms Hermon's voters, plus Remain backers, to put himself ahead of the DUP candidate.
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Ms Hermon had announced she would not be contesting the general election after nearly two decades as an MP.
Alan Chambers of the Ulster Unionist Party had been seen as the third challenger for the open seat, but he polled well short of the hunt, on 4,936 votes. Turnout in North Down was 60 per cent, with 40,643 valid votes cast.
Speaking after the win, Mr Farry said it was a “huge result for the Alliance Party in North Down and across Northern Ireland”. “The North Down area wants to remain. We believe there is no such thing as a good or sensible Brexit; indeed, all forms of Brexit are damaging to the UK, and particularly to us in Northern Ireland.”
Mr Farry said it appeared, sadly, that Boris Johnson’s Conservative majority meant the UK would be leaving the European Union. “The UK is not united on this; it is very divided on this. Northern Ireland still stands up for Remain, and if he is determined to push ahead with a hard Brexit that will have massive implications and will be very destabilising.”