BRUSSELS — European Union officials vented frustration with Britain on Friday, before negotiations next week that are aimed at smoothing the way for a favorable trading relationship after the country’s scheduled departure from the union in March 2019.

With the clock ticking, hopes are fading that Brussels and London will make enough progress by an October summit to move discussions to the topic Britain cares about most: continued access to Europe’s single market and its 500 million consumers.

In a briefing for journalists, European Union officials ticked off a list of issues that they said remained unresolved before the talks are supposed to start Monday afternoon. They requested anonymity, as is routine while discussing the content of formal negotiations.

Britain had so far failed to present sufficiently detailed plans to preserve peace in Northern Ireland, and a common travel area with the Republic of Ireland, they said. They called a pledge by London to protect the rights of more than 4 million citizens of European Union countries living in Britain, and British citizens in other member states, insufficient.