Days ahead of a summit once seen as the moment Britain and the European Union would have to reach a Brexit deal, both sides are still staring at each other over the question of the Irish border, refusing to blink.

A flurry of diplomatic meetings over the weekend had raised hopes for an agreement, only to be disappointed by the issue that has dogged the talks for months — how to ensure no hard border is created between the EU's Ireland and Britain's Northern Ireland once Brexit happens on March 29.

The EU has proposed keeping Northern Ireland in a customs union to avoid a hard border between it and Ireland. The fear is that such a border could revive tensions between Northern Ireland's pro-Irish Catholic and pro-U.K. Protestant communities, in which over 3,700 people died over 30 years of "troubles" ending in 1998.

Britain says it will only accept that plan if it is temporary and does not hive Northern Ireland off permanently from the rest of the U.K. in terms of customs arrangements.

British Prime Minister Theresa May's spokesman James Slack said Monday that negotiations are stuck because the EU "continues to insist on the possibility of a customs border down the Irish Sea," a move it feels will effectively split up the U.K., which is made up of England, Scotland, Wales and Northern Ireland.

The acrimony means it is almost impossible that EU leaders will reach a deal at their summit, which begins Wednesday and had long been pegged as the date an agreement should be reached by. The British and EU parliaments need to approve any deal, a process that could take months ahead of the official exit in March.

"Whether we do this week or not, who knows?" British Foreign Secretary Jeremy Hunt told reporters in Luxembourg where EU foreign ministers are meeting.

If Britain leaves the EU without an agreement on future relations, there could be chaos — tariffs would go up on trade, airlines could no longer have permits to fly between the two sides, and freight could be lined up for miles at the borders as customs checks are restored overnight.

To avoid this, the prospect of an extra meeting in November was raised, but only if there was decisive progress this week.

Irish Foreign Minister Simon Coveney admitted to being "frustrated" by the delay, saying that apart from Britain, Ireland is the country with most to lose from Brexit.

Coveney suggested that May is reneging on part of its commitment to ensure that no hard border involving lengthy customs checks and controls emerges on the Irish island.

He said Britain agreed in December and again in March that an unpopular "backstop" guarantee would remain in place until a better solution is found, but now appears to only want it used for a limited time.

"A backstop cannot be time-limited. That's new. It hasn't been there before," he said. "Nobody wants to ever trigger the backstop, but it needs to be there as an insurance mechanism to calm nerves that we're not going to see physical border infrastructure re-emerging."

Britain denied it is reneging on its December commitment to avoid a hard Irish border. "We don't resile from the commitments we have made in relation to the backstop," said May's spokesman, James Slack.

Like Britain's Hunt, Spanish Foreign Minister Josep Borrell said of a deal: "it seems that this week it will not be possible, but this week is not the end."

He said that he foresees no problems between Britain and Spain over Gibraltar.

"It's not a rock in the way," Borrell said, referring to the nickname of the British territory bordering Spain. He added that the Irish border problem is "more difficult to solve than Gibraltar."

Slovak Foreign Minister Miroslav Lajcak said: "There is no reason to panic. There is still time."

May is under intense pressure from her Conservative Party and its parliamentary allies not to give any more ground in negotiations, especially on the border issue.

May's political allies, the Democratic Unionist Party, stand ready to scuttle a Brexit deal over the Irish border issue. The party opposes any border customs checks but EU officials say that may be the only way to avoid a hard border.

DUP Brexit spokesman Sammy Wilson said "it is probably inevitable that we will end up with a no-deal scenario" because there was no agreement that would be accepted by Britain's Parliament.

In Luxembourg, German Foreign Minister Heiko Maas said he hopes "that in the end good sense will win the upper hand."

"Time is really pressing now," Maas warned.