Boris Johnson's resounding triumph will allow him to take the United Kingdom out of the European Union in January but it could spell the break-up of the union that has bound England, Wales, Scotland and Northern Ireland for centuries.

While Johnson's Conservative Party swept the opposition aside across much of England on his promise to get Brexit done, Scottish nationalists captured 48 of the 59 parliamentary seats in Scotland.

In Northern Ireland, supporters of a united Ireland won more seats than those in the province who want to remain part of the United Kingdom for the first time since the 1921 partition which divided the British north from the Irish Republic in the south.

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.

But opponents said it was Johnson's appeal to English nationalism with his promise to "Get Brexit done" at the expense of the interests of Scotland and Northern Ireland that had been instrumental in his success.

Both triumphant Scottish and Irish nationalist leaders and defeated opponents saw Johnson's victory as opening the way to the disintegration of the United Kingdom.

"Tonight we have seen that it is likely that Boris Johnson is on course to get a majority and it is clearly a good night for the SNP (Scottish National Party)," said Jo Swinson, leader of the Liberal Democrat Party who lost her own Scottish parliamentary seat to the SNP.

"Some will be celebrating the wave of nationalism that is sweeping both sides of the border."

SNP leader Nicola Sturgeon said the result was a clear mandate for a second plebiscite on Scottish independence.

Scotland, England's political partner for 300 years, voted against secession from the United Kingdom by 55 per cent to 45 per cent in 2014. But Scots backed staying in the European Union in the 2016 referendum and Sturgeon argues Brexit means they should have another say on independence.

"Boris Johnson may have a mandate to take England out of the European Union. He emphatically does not have a mandate to take Scotland out of the European Union. Scotland must have a choice over our own future," Sturgeon said.

A majority in Northern Ireland also voted against leaving the European Union and anti-Brexit parties took more seats in the province for the first time. Nationalists said the result paved the way towards a vote on whether there should be a united Ireland.

"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."

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.

Australian Associated Press