Mr. Corbyn introduced the motion in light of reports in recent days that Mr. Johnson was planning to flout the law that blocks a no-deal Brexit by refusing to ask Brussels to delay the current deadline of Oct. 31. Mr. Corbyn called that an “assault on the rule of law.”

Ministers later denied that the prime minister would break the law, but still suggested the government was looking for loopholes to avoid Mr. Johnson having to ask Brussels for an extension.

John Bercow plans to step down by the Brexit deadline.

John Bercow, the animated speaker of the House of Commons, said on Monday that he would step down by Oct. 31, the day Britain is scheduled to leave the European Union. He said he would not run should a general election be called before then.

“This has been, let me put it explicitly, the greatest honor and privilege of my life, for which I will be eternally grateful,” he said, becoming emotional as he thanked his wife and children for their support.

He also warned lawmakers to respect the process of the parliamentary system, noting, “We degrade this Parliament at our peril.”