Britain says there must be no border posts between Northern Ireland and the Irish republic after Brexit.

But it says “flexibility and imagination” will be needed to maintain the currently invisible border once the U.K. is out of the European Union while Ireland remains a member.

Britain says a future “customs partnership” could eliminate the need for border checks, but EU leaders are skeptical.

The U.K. also says it will preserve the long-standing common travel area with Ireland.

Britain is releasing Northern Ireland proposals Wednesday, one in a series of papers covering thorny issues in Brexit negotiations, which are due to resume in Brussels at the end of this month.

The Irish government welcomed Britain’s commitment to “avoiding a hard border.”