Sebastian Mallaby, author of “The Man Who Knew: The Life & Times of Alan Greenspan,” is the Paul A. Volcker senior fellow for international economics at the Council on Foreign Relations and a contributing columnist for The Post.

After years of exuberant argument — spirited, obstinate and frequently dishonest — Britain is about to leave the European Union. The deadline looms in less than six months, and the entrance to the last off-ramp is narrow. Meanwhile, a second political shock has struck. It may matter more than the first one.

This second shock involves the opposition Labour Party, which has cartwheeled from Clintonian Third Way centrism to Marxian hard leftism in less than a decade. At its recent party conference, Labour promised not only to nationalize industries such as water, but also to create boards featuring local politicians, workers, environmentalists and consumers to oversee their management. Labour’s official platform demands that companies with 250 or more employees put 10 percent of their shares in “inclusive ownership funds” overseen by workers — presumably these shares would have to be confiscated from current owners. Economists linked to the party are mulling a reinstatement of capital controls and a four-day workweek.

Labour is polling almost as strongly as the ruling Conservatives. If the unpredictable Brexit process were to result in an early election, the outcome would be a toss-up. Even if the next election happens on schedule, in 2022, allowing the Conservatives a chance to heal their internal Brexit divisions, Labour’s chances of victory might stand at 1 in 3 or so. That is a far higher probability than most analysts attach to a disorderly Brexit, in which Britain’s borders and supply chains are suddenly disrupted. When London’s captains of industry mutter about quitting Britain, the specter they fear most is not Brexit but a Labour government.

Britain has a reputation for pragmatism and moderation. Unlike France or Italy, it has never had a serious communist party. With the notable exception of Margaret Thatcher, its postwar Conservative prime ministers would, in American political terms, be classed as centrist Democrats. But it has also had periods of radicalism — not just Thatcher’s pro-market overhaul, but the sweeping nationalization under the Labour government of 1945-1951 — and it appears to have entered another radical phase now. The ruling Conservative Party is bent on delivering Brexit, a costly experiment in deglobalization. The Labour Party is committed to a sort of high-tax collectivism.

Why is this happening? Some obvious theories are not fully convincing. Inequality in Britain was stable between the early 1990s and 2008 and has declined since then. Median wages for employees fell sharply after the financial crisis, but have been recovering since 2014. As a percentage of the working-age population, the employment rate is the highest ever. Even male workers, who have borne the brunt of deindustrialization, are enjoying the best employment rate in a quarter of a century.

So whence Britain’s radicalization? One answer is that respectable nationwide averages mask the tough experiences of subgroups. Between 2010-2011 and 2014-2015, more than a third of Britons suffered a fall in household income of more than 5 percent. Those depending heavily on the state have suffered the effects of harsh cuts in government spending. Young workers, who have provided much of the support for Labour’s lurch leftward, have fared worse than old ones. The shocking price of homes has made it difficult for non-owners to live in regions with good job prospects.

But the larger explanation is cultural. On the left and the right, there is a sense that society is changing rapidly and unfairly. The right-wing version of this insecurity centers on migration. Between 1964 and 1989, the number of migrants arriving to live in Britain never exceeded 250,000 in a year. But in the two years leading up to the Brexit vote, new arrivals exceeded 600,000 annually. When Conservative populists rant xenophobically about “taking back control,” they are exploiting the backlash from that surge in foreign voices.

Meanwhile, left-wing cultural insecurity centers on a form of inequality that goes beyond the data on incomes, encompassing ownership of assets and the sense of job stability. The haves can count on a salaried job, a stake in the housing market and perhaps some corporate shares or share options. The have-nots face short-term work contracts, high rents and no prospect of a stake in the profits generated by capitalism. When the Labour Party proposes “community control” of companies and a redistribution of their stock, it is tapping into this frustration.

For onlookers outside Britain, the radicalization of a once sensible and moderate political culture should stand as a warning. Globalization and technological change, which are about to intensify thanks to breakthroughs in artificial intelligence, generate similar insecurities across the mature democracies. What Britain teaches is that when centrists fail to address these challenges, populists of left and right will fill the void. The race is on to define a political program that assuages real grievances without damaging prosperity or decency.

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