LONDON - British Prime Minister Theresa May came under mounting pressure on Monday after talks between London and Brussels reached a dramatic stand-off at weekend over how to prevent the return of a hard border between Northern Ireland and the Republic of Ireland after Britain's departure from the European Union (EU) in March 2019.

May, who is travelling to Brussels for a key EU summit on Wednesday, sent Dominic Raab, the British Brexit secretary, to meet with the EU's chief negotiator Michel Barnier on Sunday for unscheduled talks about "key issues" including the Irish border.

After the two sides failed to reach an agreement at the end of the one-hour negotiations, no further talks are scheduled for them ahead of the Wednesday summit.

Speculation that a Brexit deal had been reached between the two sides was quickly denied by Barnier, raising the prospect of a complete breakdown of the Brexit process.

Raab told Barnier that the British prime minister could not sign up to the current terms for Brexit at the European Council meeting on Wednesday. In fact, both sides hoped to sign off on a draft withdrawal agreement setting out the terms for Britain's exit.