International policymakers are ramping up their warnings of the dangers of a British exit - popularly known as "Brexit" — from the political and economic alliance that has united Europe for the past four decades. Voters in Britain will decide whether to leave or remain in the European Union in a referendum on Thursday, but financial market volatility has already spiked as polls show a growing desire to abandon the partnership.

The decision carries hefty consequences for American businesses, which employ more than a million people in Britain. The United States is the largest single investor in Britain, and many firms consider it the gateway to free trade with the 28 nations that make up the E.U. Brexit would jeopardize their access to those markets, potentially reducing revenue and forcing some firms to consider relocating their European operations elsewhere. That has put corporate America onto the front lines of the campaign to keep the union together, with several of Wall Street's biggest names donating substantial sums to the effort.

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