Brexit Minister Dominic Raab (L) and Brexit negotiator Michel Barnier (R) hold a joint press conference in Brussels, Belgium on July 18, 2018. Dursun Aydemir | Anadolu Agency | Getty Images

The European Union's top Brexit negotiator says urgent talks with Britain's point person did not result in their reaching agreement on outstanding issues. EU negotiator Michel Barnier said: "Despite intense efforts, some key issues are still open" in the divorce talks between the European Union and Britain. Barnier and his British counterpart, Dominic Raab, met in Brussels for surprise talks on Sunday. The discussion prompted rumors that a full agreement might be imminent, but Barnier says the future of the border on the island of Ireland remain a serious obstacle. He says the need "to avoid a hard border" between Ireland and the U.K's Northern Ireland is among the unsettled issues. An EU official says no further negotiations are planned before an EU leaders summit on Wednesday.

The "Irish backstop" is the main hurdle to a deal that spells out the terms of Britain's departure from the EU and future relationship with the bloc. After Brexit, the currently invisible frontier between Northern Ireland and Ireland will be the U.K.'s only land border with an EU nation. Britain and the EU agree there must be no customs checks or other infrastructure on the border, but do not agree on how that can be accomplished. The EU's "backstop" solution — to keep Northern Ireland in a customs union with the bloc — has been rejected by Britain because it would require checks between Northern Ireland and the rest of the U.K. The alternative — to keep the entire U.K. in a customs union until a permanent solution can be found — has outraged pro-Brexit members of British Prime Minister Theresa May's government, who claim that approach would limit the country's ability to strike new trade deals around the world.

The idea is also anathema to the Democratic Unionist Party, a Northern Ireland Protestant party that props up May's minority government. So even if May strikes a deal with Brussels, she will struggle to get it past her government and Parliament at home. Raab's predecessor, David Davis, wrote in the Sunday Times that May's plans for some continued ties with the EU even after Britain leaves the bloc is "completely unacceptable" and must be stopped by her ministers. May is struggling to build a consensus behind her Brexit plans ahead of a Cabinet meeting Tuesday that will be followed by Wednesday's EU summit. If Davis' call for a rebellion is effective, the Cabinet meeting is likely to be fractious.