Northern Ireland elected more Irish nationalists to Britain's parliament than pro-British unionists, a first since the partition of Ireland in 1921.

Nationalists said the result paved the way towards a vote on whether there should be a united Ireland, as Prime Minister Boris Johnson won a crushing victory across the union.

"We are heading towards a border poll, I can't give you a definitive date, but we need to do the spade work now and prepare ourselves," Sinn Fein leader Mary Lou McDonald said.

"We need to, in an orderly fashion, structure the conversation about a new Ireland and constitutional change. I don't think unionism should be alarmed or frightened, this is a huge opportunity for everybody who lives on this island."

Sinn Fein gains

The largest nationalist party Sinn Fein narrowly won the final of the region's 18 seats. That meant Sinn Fein kept its seven seats and the resurgent fellow pro-Irish SDLP won two, combining to pass the Democratic Unionist Party, which fell from 10 seats to eight.

The cross-community Alliance Party took the final seat, adding to the majority of anti-Brexit MPs after its share of the total vote rocketed by almost 10%.

Throughout the election campaign, Johnson said he was committed to the union and denied accusations that his Brexit deal would create an economic barrier between the British mainland and Northern Ireland.

"At this stage it does look as though this one-nation Conservative government has been given a powerful new mandate, to get Brexit done and not just to get Brexit done but to unite this country and take it forward," Johnson said on Friday after winning his own seat in west London.

'Betrayal'

One of those to lose their seat was Nigel Dodds, deputy leader of the Democratic Unionist Party which propped up the minority Conservative government in London after the last election in 2017.

The DUP said it had ultimately been betrayed by Johnson's Brexit deal with Brussels which they said would create an economic border between Britain and Northern Ireland.

"The great irony of all of this is that for decades unionists have looked over their shoulders and decided that Irish nationalists were the great threat ... but actually it's English nationalism," Mike Nesbitt, former leader of the Ulster Unionist Party, told the BBC.

Nationalists also won an overwhelming victory in Scotland, prompting calls for a second independence referendum in the province.

READ MORE: New independence vote looms in Scotland after nationalists' crushing victory

'Pro-remain alliance'

In some of Northern Ireland's constituencies, pro-remain alliances were concluded ahead of the election. In some cases, the Greens joined hands with the nationalists.

"At extraordinary times, we make extraordinary decisions, and that's why we stood down in North and South Belfast, in order to maximise the pro-remain vote," Malachy O'Hara, deputy leader of the Northern Ireland Green Party, told Euronews.

"The majority of people here voted to remain and they see Brexit as something that is very unfairly being imposed upon them," Sinn Fein's John Finucane told Euronews ahead of the vote.

Finucane managed to unseat Dodds in the DUP stronghold of North Belfast.