"EU-UK negotiations have entered the endgame, time is running out quickly," said an EU diplomat after the bloc's chief negotiator Michel Barnier gave member states' envoys to Brussels a downbeat assessment of the state of play, Reuters reported.
"It is for the UK to choose between ... a positive outcome or a no deal outcome."
With growing fears of "no-deal" chaos after London finally leaves the EU's orbit on Dec. 31, talks resumed before British Prime Minister Boris Johnson and European Commission President Ursula von der Leyen review the situation in a call.
Irish Prime Minister Micheal Martin, whose country would be the hardest hit of the 27 EU states if there is no trade accord, put the chances of a deal at 50-50. Investment bank JPMorgan said its odds on a no-deal had risen to one third from 20%.
The British pound tumbled on concerns that there would be no agreement covering annual trade worth nearly $1 trillion.
Barnier told members of the European Parliament in a separate briefing that negotiations could go on until Wednesday, but no further.
EU diplomats said the ball was now in Johnson's court.
"People need to understand that the British are playing with fire here and the fire can burn everybody and that's something we should all try to avoid," said Mairead McGuinness, Ireland's commissioner in the EU executive.
However, the Sun newspaper reported that Johnson, a figurehead for Britain's campaign that led to a 'vote leave' victory in a 2016 referendum, was ready to pull out of the talks within hours unless Brussels changed its demands.