“This is now a rupture within the fabric of the Conservative Party,” said Alan Wager, a research associate at The U.K. in a Changing Europe, a research organization.

Even onetime allies of the anti-Europe absolutists in Parliament now blame them for what could turn into a generational slump in support for Conservatives. And no one currently speaks more loudly for them than Mr. Francois.

“It’s morphing from a battle between Leave versus Remain into a battle between the people and the establishment,” Mr. Francois said in an interview last week in his Westminster office, where a life-size target from a military firing range stood in the corner. “And the people increasingly just want to get out of Europe, and the establishment want to keep us in.”

With Britain’s divorce from the European Union knocked back until as late as October, some fear that Brexit — at least the complete break that the self-described “Spartans” in Parliament champion — may inexorably be slipping away. Lawmakers generally concede that Parliament would sooner reverse Brexit than let Britain crash out of Europe without a deal.

That has so dispirited some Brexiteers that they say they have given up on leaving the European Union at all. But it has also set the stage for the hardest of the hard-core pro-Brexit lawmakers, like Mr. Francois, to mount a high-profile campaign against what they describe as months of subterfuge by pro-Europe elements in Mrs. May’s office, the Civil Service and the news media, among others.