London, Dec 13 (AFP) Prime Minister Boris Johnson may be revelling in his mighty election victory but the results in Scotland and Northern Ireland have hinted at battles ahead in trying to keep the United Kingdom together.

Strong performances by Scottish and Irish nationalists in Thursday's snap vote will increase concerns about independence movements gaining momentum north of England's border and in Northern Ireland.

The Scottish National Party (SNP) won 48 seats, almost matching its performance in the 2015 election and setting up a showdown between Johnson and its combative leader Nicola Sturgeon.

Sturgeon said it had been an "exceptionally good night" for her party, leaving Scotland and the rest of Britain on "divergent paths".

"I accept that Boris Johnson after this election has a mandate to take England out of the European Union, but he does not have a mandate to take Scotland out of the European Union," she said, adding she would push for another independence referendum after losing a 2014 vote.

"I have a mandate, a renewed, refreshed, strengthened mandate, to offer people in Scotland the choice of a different future.

"I don't pretend everybody who voted SNP yesterday will necessarily support independence, but there is a clear endorsement Scotland should get to decide our future and not have it decided for us," she said.

Sturgeon is expected to write to Johnson before Christmas to formally demand Holyrood -- the seat of the Scottish government -- be given the power to hold a second independence vote.

But Johnson's Conservative government is unlikely to accept her demands, and has little incentive given its massive majority.

Conservative party chairman James Cleverly told the BBC that Thursday's result was not a mandate for a second independence vote.

"The majority of votes in Scotland went to parties that are pro-union," he said. "The 2014 referendum was meant to be a once in a generation referendum." In that poll, Scotland -- a country of 5.4 million -- voted by a margin of 55 per cent to 45 per cent to stay in the United Kingdom.

"The SNP, before they revert back to these calls for another referendum, should probably think carefully about how they sort out (healthcare) in Scotland and how they reverse the slipping standards in Scottish schools." Johnson is also facing the serious but less urgent threat of keeping Northern Ireland part of the United Kingdom, in the wake of demands by nationalists for a united Ireland.

Both Scotland and Northern Ireland voted against Britain's departure from the EU in the shock 2016 referendum.

And Brexit has raised concerns that a possible "hard border" with EU-member Ireland to the south could lead to a return to the tensions and bloody sectarian violence of the past.

Nationalists now -- for the first time since the partition of Ireland in 1921 -- hold more seats in the British parliament than those who want to stay part of the United Kingdom.

The hardline Democratic Unionist Party, which previously held most Northern Irish seats in the British parliament and propped up the Conservative government, had a tough night, losing deputy leader Nigel Dodds.

Edwin Poots, Dodds' colleague in the devolved Northern Ireland assembly, said the deputy leader's defeat was "very damaging for unionism".

"Ultimately if we are going to protect the union, enhance the union and secure the union, then we're going to have to have people voting unionist," Poots told BBC Northern Ireland.

Unionists have also warned that Johnson's Brexit deal creates a customs border down the Irish Sea, which they fear nationalists will exploit to pull Northern Ireland away from London's orbit. (AFP)

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