It is a measure of how far British politics has passed through the looking glass that MPs feel compelled to put in law that prime ministers should govern with the consent of parliament. There was a time, not too long ago, when that sort of thing was understood as a convention underpinning democracy. The imminent arrival of Boris Johnson in Downing Street has made it prudent to get implicit constitutional principles down in writing.

The amendment that passed today would not forbid Johnson from dissolving the House in order to get Britain out of the EU without a negotiated deal. It is a more convoluted device, attached tenuously to a passing bill pertaining to Northern Ireland. It instructs the government to report to the Commons on a timetable that would preclude prorogation. Crudely speaking, it wedges the parliamentary door open in the run-up to the 31 October Brexit deadline – in case Johnson plans on shutting the legislature down to facilitate a desperate dash for no deal.

MPs pass amendment seeking to thwart no-deal Brexit prorogation Read more

There is no sign that he wants to take such an extreme measure, although he hasn’t ruled it out. And the 41-vote majority for the door-wedge amendment can’t be read as a measure of opposition to a hard Brexit. It is just a warning not to torch parliamentary democracy along the way. It is also a function of political interregnum. Theresa May’s administration has expired. Johnson’s is not yet formed. Under those circumstances it was easier for cross-party sponsors of the amendment to recruit Conservative rebels. One minister, Margot James, resigned from the frontbench to back the move, but there isn’t much of a government to resign from anyway. More revealing was the scale of cabinet-level abstention. Philip Hammond was among those who found reason to be elsewhere. Notable, too, is May’s refusal to issue any kind of rebuke to her demob-happy colleagues for ignoring the government whip.

No one imagines that Hammond is holding his breath in anticipation of a plum job offer from Johnson, nor that he wants one. It is still a punchy move, though, for the serving Tory chancellor to signal encouragement to a rebellion aimed at wounding a Tory prime minister before he has even taken office.

This demonstration of the fragility of the government’s majority is significant in practical and symbolic terms. It is a reminder that the political roadshow around Brexit has been on a weird repertory tour around the country for the duration of the Tory leadership contest. The action has been in town halls, playing to audiences of Conservative members who flock to hear Boris’s greatest hits. But eventually the show comes back to the main Westminster stage, where Johnson is not such an adored star and where numbers matter more than words. Johnson’s verbal flamboyance won’t change the tally of MPs who think it would be reckless folly for Britain to shred existing agreements with its continental allies. And it is easily forgotten that completing Brexit is not simply a matter of passing one deal by one vote on one day. It is a legislative odyssey. Even in the no-deal scenario, parliament would have to vote to approve money for contingency measures (and ratify the emergency agreements that a panic-stricken prime minister would cobble together in Brussels on the most disadvantageous terms to keep borders open). Johnson has taken a long warm bath in his fans’ adoration. This vote should serve as a cold shower of reality.

But there is a second, more profound problem for the next prime minister exposed by a show of rebellious intent in the Commons. By constitutional principle, Downing Street is occupied by the leader who commands a majority in parliament. That entitlement was won comfortably by David Cameron in 2015 and only barely held by May in 2017. The mandate is already second-hand and ragged. Johnson will arrive in office by a different route, never before trodden in British history. He will be installed by a vote of party members to the role of Tory leader with an automatic right of upgrade to prime minister, based on the presumption that his predecessor’s majority – already flimsy and unreliable – is still sound. There will be no evidence to support that belief until Johnson tests it. Unless he can prove that his government meets the essential constitutional qualification, his legitimacy, and his right to be in Downing Street at all, will be in doubt.

Johnson has said he has no intention of calling an election and it is easy to think of reasons to avoid one. But it is also hard to see how he will function in office without a reliable majority, especially if he wants to pursue a Brexit course that is certain to stir obstructive and rebellious spirits in the Commons. He is cruising to victory thanks to a devoted following among his party’s members. But they are not the audience he has to impress after next week. Johnson should soak up their applause as it rings in his ears now, because the Commons has today reminded him that it is not going to be an easy crowd to please.

• Rafael Behr is a Guardian columnist