The plan, referred to as “two borders for four years,” would pull Northern Ireland out of the European customs union but leave much of its economy aligned with E.U. regulations for a period.

Reaction: Jean-Claude Juncker, the European Commission president, welcomed parts of the plan but found others problematic. Irish officials, much less optimistic, objected to required checkpoints between Northern Ireland and the south, and to the proposal that the north could vote to leave the arrangement in four years.

Another ‘prorogation’: Downing Street also announced plans to prorogue, or suspend, Parliament again, despite a stinging defeat in the Supreme Court that overturned Mr. Johnson’s previous attempt to do so. This one has a better shot, and would be brief.