Editor's Note: This story was published before the UK referendum.

If Britain does end up exiting the European Union, large U.S. banks can at least say they were prepared for it.

Resolution plans big banks were required to put in place following the financial crisis have made them expert at contingency planning, so if they have operations in Britain and other European markets, they have likely modeled for any and all types of disruptions.

Bill Rhodes, a retired senior international officer at Citigroup,  said that since the financial crisis, U.S. banks have been required to undergo a more rigorous contingency-planning process than their European counterparts. Some even modeled for Britain leaving the EU in their recent stress tests — even though U.S. regulators didn't require them to do so, according to news reports.

Otherwise, Britain's potential exit from the EU is anything but good news for large U.S. banks.

Economic uncertainty created by the so-called Brexit would likely weaken capital markets activity in Europe, crimping profits at such firms as JPMorgan Chase, Citigroup and Bank of America, all of which have substantial overseas operations.

The banks' costs, too, could creep up as they may be forced to shift jobs out of Britain and into other European markets. JPMorgan Chairman and Chief Executive Jamie Dimon has already warned that his company could move thousands of jobs out of the country if Britain exits the EU.

"It is my opinion that this is a terrible deal for the British economy and jobs," Dimon said in early June, when he traveled to the company's offices in Bournemouth, England, to warn about the potential effects of Britain's exit from the European Union.

In a town hall-style meeting, Dimon said that a vote in favor of an EU exit would create years of economic uncertainty in Britain and cause companies to think twice about doing business there. He suggested, too, that other European countries might choose to stop trading with Britain in retaliation for abandoning the 28-member EU.

Other prominent business leaders have issued similar warnings in recent weeks — yet many Brits remain unconvinced. Citizens will go to the voting booths Thursday to decide whether Britain should remain in the EU or go it alone, and the latest polling data has the vote as being too close to call.

Even banks with little to no dealings in Europe would likely feel some short-term pain if Britain opts out of the EU. Just the prospect of such a scenario has caused already-rock-bottom interest rates to fall even further in recent weeks, pressuring banks' already razor-thin margins.

"It is causing the market to be risk averse — and that's why I think you're seeing long-term rates go down," said Kevin Barker, an analyst with Piper Jaffray.

Large banks, though, would appear to have the most to lose, analysts said.

For years, banks here have used the U.K. as an entry point for their EU operations. Having a British subsidiary gave companies "passport" privileges to operate freely across the EU bloc.

JPMorgan, for instance, earned $7.7 billion in operating income in 2014, with most of it coming from the firm's broker-dealer business, according to Keefe, Bruyette & Woods. Bank of America and Citigroup earned $5.2 billion and $4 billion, respectively.

Overall, a Brexit vote could result in a 3% decline in full-year 2016 earnings for JPMorgan, Bank of America and Citigroup, according to KBW. Estimated 2017 profits, post-Brexit, look even worse, with earnings per share projected to decline between 5% and 7%, KBW said in a report last week.

The stakes are highest for JPMorgan because the company's European operations are heavily concentrated in the U.K., analysts said.

The bank employs about 16,000 people, or about 7% of its total workforce, in the U.K. During his town-hall speech, Dimon said JPMorgan could shift as many as 4,000 jobs away from the U.K., should a Brexit vote come to pass.

Citigroup has also warned of a potential "rebalancing" of its EU operations.

On a more positive note banks, it would take at least a couple of years for Britain to exit the EU, giving banks time to adjust to the new legal and political landscape, said Joo-Yung Lee, head of North American Financial Institutions at Fitch Ratings. She said she would expect banks to "pivot" away from the U.K. and set up shop in other European markets.

But some clients may not want to wait for U.S. banks to shift operations and could opt to move their business to more established European players, like Deutsche Bank, said KBW's Brian Kleinhanzl.

In a research note, Kleinhanzl said that German and French banks are particularly well positioned for Brexit. "These banks could take market share from the U.S. banks during the two-year transition period; although we'd expect the U.S. banks to regain any lost ground over the long term," he wrote.

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