There’s a new potential fissure in the European Union. This time it isn’t Greece teetering on the edge of departure, but Great Britain.

On June 23 Brits will vote on whether or not to leave the EU, the so-called “Brexit.” It’s a big deal — especially for businesses and investors across the continent. According to Bank of England Governor Mark Carney, leaving the EU is Britain’s “biggest domestic risk to financial stability.”

Since 1975 Britain has been part of the EU single market, in which there are no taxes, tariffs or quotas on trade. Goods, services, money and people also move freely between EU countries. British citizens can start a business in Spain or take a job in Germany, or vice versa. And over 40 years, these economies have gotten quite entangled. Today, Britain exports 45% of its goods and services to the rest of the EU. More than 2 million non-British Europeans work in Britain.

So why the potential divorce? Prime Minister David Cameron pledged to hold a referendum on staying in the EU during a re-election bid last year. He was looking to shore up political support on the right and appease members of his Conservative Party who demand that Britain negotiate its membership terms in the EU.

So now there are two major campaigns underway. They boil down to this: The “remain” campaign, led by Cameron, argues that EU membership has made Britain economically successful. The “leave” side, led by the justice minister and the former mayor of London, argues that EU laws and regulations have stripped Britain of its sovereignty and control of its borders.

Pro-Brexit voters appear particularly motivated by immigration concerns: 333,000 people moved into Britain in 2015, the second-highest figure on record — ironically drawn there by its comparatively healthy economy. Brexit proponents also claim that Britain will be better off rid of the EU’s supposedly costly regulations. (But it also will have to form a host of new trade agreements, which guarantees years of financial uncertainty for businesses.)

Most economists see lots of downside to a Brexit, including market instability, recession, higher unemployment and a weakened pound. Finance ministers of the G20 countries collectively agree that severing Britain from the EU wouldshock the global economy.

If the referendum passes, a withdrawal agreement still must be ratified by the British Parliament. Undoing the popular vote, however, would constitute political suicide for MPs.

The vote could be close on June 23. Recent polls show 41% in favor of leaving, 46% in favor of staying and 12% undecided.