London — FOR a sense of how historically important Thursday’s referendum result in Britain is, consider the following. The last time British voters were formally consulted on continued membership of what was then called the Common Market, in 1975, mainstream politicians of the major parties joined forces with experts and establishment figures to persuade Britons that leaving was too great a risk. The “In” campaign prevailed.

This time, 41 years later, an almost identical strategy was adopted by the Remain camp. The Conservative prime minister, David Cameron, led the effort, with the notional support of the Labour Party leader, Jeremy Corbyn, and a phalanx of heavyweight economists and world leaders (including, most theatrically, President Obama). Open letters signed by dozens of accomplished figures in their respective fields urged the voters to shun the perils of Brexit.

In 1975, the British people deferred to such authorities. In 2016, they ignored them.

It is too early to say precisely why the Leave campaign prevailed on Thursday, although initial analysis by the pollster Lord Ashcroft suggests that there were three primary reasons voters put an X beside Brexit: first, Lord Ashcroft proposed, “The principle that decisions about the U.K. should be taken in the U.K.”; second, “Voting to leave offered the best chance for the U.K. to regain control over immigration and its own borders”; and third, “Remaining meant little or no choice about how the E.U. expanded its membership or powers.”

Beneath these specific imperatives lurked more general sentiments — in particular, a contempt for the political class and a disdain for the self-styled aristocracy of experts. However much the Remain camp warned of the economic risks, the voters seemed more animated by the politics of identity and by the conviction that no government could control immigration while Britain was a member of the European Union.

For decades, the economy has been the principal issue in the polling booth, according to a scale of priorities evidenced as recently as the general election of May 2015, when Mr. Cameron beat Mr. Corbyn’s predecessor, Ed Miliband, with a promise to finish the job of repairing the nation’s finances. But this vote was different. Economic peril was emphatically less important than the European Union’s lack of accountability (its famous democratic deficit) and the British government’s failure to follow through on promises to restrict immigration.

This campaign has been pockmarked by xenophobia and thinly concealed racism: a deeply disturbing development in mainstream politics. True, there was widespread disgust at the poster introduced a week before polling day by Nigel Farage, leader of the reactionary U.K. Independence Party, which depicted hordes of migrants — mostly of apparent Middle Eastern origin — as if lining up to enter Britain under the label “breaking point.” But Thursday’s referendum result was every bit as much Mr. Farage’s victory as a triumph for Vote Leave, the more genteel campaign whose leading figures included Boris Johnson, the former London mayor and now favorite to succeed Mr. Cameron as party leader, and Michael Gove, the Conservative justice secretary and, until recently, one of Mr. Cameron’s most trusted lieutenants.

The prime minister knew it was possible that he would lose the referendum and had drawn up plans in case his own members of Parliament demanded a vote of confidence in him. A few weeks ago, one of Mr. Cameron’s closest allies walked me through the strategy, which, this person assured me, would place the prime minister’s position beyond doubt and enable him to retire from leadership at a time of his choosing.

The unforgiving light of Friday morning made a nonsense of such plans. Having lost the referendum, Mr. Cameron could scarcely stay in Downing Street. All authority had drained to a government within a government, run by the Johnson-Gove axis. How could Mr. Cameron plausibly negotiate Britain’s exit from the union with our soon-to-be-former European partners when this was an outcome he had resolutely opposed during the campaign? As he weighed his options, it became obvious he had to go.

As a student of political change, Mr. Cameron also knew that — in a single night — his cohort of followers had been supplanted. The Conservative Party, formerly managed by the prime minister’s modernizing clique, is now wholly owned by the Brexiteers, whose direction is radically different.

The so-called mods who took over in 2005 when Mr. Cameron swept to the leadership sought to make the party more in step with today’s world, more pluralist in perspective, more environmentally aware and more sympathetic to women, gay voters and ethnic minorities. The Brexiteers, in sharp contrast, are the ideological grandchildren of the Thatcherites, longing to “get out of Europe” and convinced that Britain’s future is as an independent, buccaneering nation of entrepreneurs, unhampered by the wet cement of communitarian obligations.

They see the sun finally setting on the British Empire and applaud its descent. For them, small is beautiful. That goes for the state, too, by the way. Those voters who were taken in by Leave’s apparent promises to fund health care services with the £350 million (about $480 million) no longer supposedly sent every week to the European Union are in for a shock.

As the Conservative Party now metamorphoses, Labour has comprehensively abandoned the social-democratic center-ground from which it won three successive elections between 1997 and 2005. Indeed, Mr. Corbyn’s followers and the Brexiteers have more in common than they would care to admit: a shared antipathy to the old political elite, crony capitalism and the sheer pace of global change.

In leaving the world’s largest single market, Britain has resigned from the grown-ups’ table and effectively kicked out a prime minister voters had re-elected only 13 months earlier. As tantrums go, this was Olympic-class. So the referendum’s winners had better deliver what they promised — and fast. The languorous talk of taking the process of exit slowly and cautiously is deluded. Having voted for change, the electorate will expect it quickly. Having demanded that Britain “take back control” of immigration, they will not forgive backsliding.

This is a new era of foot-stamping urgency and vexed intolerance. No wonder Donald J. Trump relished the outcome as he landed in Scotland to promote one of his golf courses. This is what Britain voted for on Thursday. For the bleak time ahead, we Britons have only ourselves to blame.

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