Diplomats said talks in Brussels led by British Prime Minister Theresa May's chief lawyer, Geoffrey Cox, failed to yield a repackaged deal on Tuesday, with barely over three weeks to go before Britain's scheduled exit on March 29, Reuters wrote.
Talks by lower-ranking officials were expected to continue on Wednesday but there was no clear schedule yet for further negotiations between Cox and the EU's Michel Barnier.
"(EU Brexit negotiator) Michel Barnier has informed...that while the talks take place in a constructive atmosphere, discussions have been difficult," said Margaritis Schinas, spokesman for the European Commission, the bloc's executive.
"No solution has been identified at this point that is consistent with the withdrawal agreement, including the protocol on Ireland and Northern Ireland, which will not be reopened," Schinas told a news briefing after Barnier updated the Commission.
At the heart of the deadlock is the Irish "backstop", an insurance policy the EU wants to ensure no return to a hard border between the British province of Northern Ireland and EU member Ireland after Brexit.
British Attorney General Geoffrey Cox said on Wednesday that Tuesday's discussions in Brussels were "robust" and detailed, and would be resuming soon.
Diplomats speculated that, if EU and UK negotiators could seal a deal over the weekend, May could come to Brussels on Monday to give it political endorsement and take it back to London just a day before the House of Commons votes on it.
"It's unlikely there would be a deal before the weekend," an EU official said. "We are preparing for a working weekend."
"We are at a standstill," said a national EU diplomat who follows Brexit.
Britain is currently set to depart the EU on March 29 under a timetable put into legislation by May's government in 2017.
The EU's 28 leaders including May will decide at a March 21-22 summit whether to extend the Article 50 negotiating period beyond March 29. "How long an extension will depend on the House of Commons vote," the diplomat added.
The bloc will be watching the British parliament's action next week but few in EU hub Brussels believed the deal would be ratified by deeply split UK lawmakers in their second go at it - two months after they resoundingly rejected the package.