Whatever Mr. Johnson’s action means for British democracy, it has fortified a growing belief in Brussels that he is not really gunning for a no-deal Brexit. Instead, many believe that he is aiming to nail down a modestly revised withdrawal agreement, one he can get a shocked Parliament to approve before Oct. 31, the day that he has vowed Britain will leave the European Union, deal or no deal.

In effect, Mr. Johnson is trying to play hardball with both Parliament and Brussels, diplomats say, insisting that Oct. 31 is a drop-dead date for a Brexit deal and trying to use the newly compressed timetable to force through previously unconsidered compromises.

Even as he vows that Britain will leave the European Union on Oct. 31, he is intensifying discussions with Brussels through his Brexit negotiator, David Frost, to find a workable alternative to the infamous backstop — the guarantee against a physical border between Northern Ireland and Ireland after Brexit that many believe would threaten the peace agreement in the North.

[What is the Irish backstop, and why is it holding up Brexit?]

Critical national leaders like Chancellor Angela Merkel of Germany have expressed a willingness to consider practical, workable alternatives to the backstop, even amid widespread skepticism that any can be devised so quickly, if at all.

At the same time, both London and Brussels insist that their contingency plans for a no-deal Brexit are well advanced and will be ready to absorb much of the shock, assertions that are widely discounted. Most analysts expect considerable economic damage from a cliff-edge split, with major disruptions in trade and commerce, and with Britain suffering the most.