A new analysis has found that the U.K.'s decision to exit the European Union has already slowed the nation's economic growth by a cumulative 2.1% as of the first quarter of 2018, compared to likely results had Brexit been rejected two years ago. The study also found that the referendum vote is now costing the British government ?440 million ($584 million) per week in lost tax revenue, challenging longtime claims by Brexit supporters that leaving the E.U. would improve government balance sheets.

The study is from the pro-E.U., London-based Center for European Reform.

Gauging the economic impacts of a single policy is very difficult, because it's impossible to perform a truly scientific experiment on complex, real-world systems. But the study's methods, which compared the U.K.'s growth to a weighted basket of 36 comparable economies, are compelling on their own merits.

Using a computer model, the think tank created a "synthetic U.K." that accurately tracked to 20 years of historical growth. But real growth fell short of the model's projections starting immediately after the Brexit vote, suggesting a likely - though not inarguable - culprit for the lag.

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Brexit is not law yet. But businesses are already preparing for it by shifting supply chains and relocating operations, and uncertainty about ongoing negotiations may be discouraging investment. Britain's economy has lagged so severely, even in the midst of a broad global expansion, that its first quarter growth of 0.1% puts it behind Italy and makes it the slowest-growing economy in the G7.

CER’s analysis is politically explosive because advocates for the U.K.'s exit from the E.U. argued that it would produce a "Brexit dividend" -- savings and increased government revenue that could be used to, among other things, funnel additional funds into Britain's National Health Service. That outcome seems increasingly unlikely, since CER finds that lost tax revenues are now significantly higher than the ?350 million per week in payments to the E.U. that Remain partisans wanted to reclaim. The CER study's budget findings broadly align with those of other studies, including one from the U.K. government's own Office for Budget Responsibility.

The argument also casts a grim light on U.S. President Donald Trump's accelerating 'trade war' with both China and allied economies. As with Brexit, an overwhelming majority of professional economists disagree with Trump's blustery skepticism of free trade, even with allied neighbors. The U.S. stock market hasn't yet responded decisively to Trump's escalating tariffs, but the example of Great Britain could serve as a dire warning of the potential damage.