US companies and sectors that are most exposed to a Brexit

They may operate thousands of miles from British voting booths for the most part, but a lot of U.S. companies have something to lose if the UK votes to leave the European Union next week.

Take the banks, for example. U.S. financial institutions have traditionally set up shop in Britain as a gateway to the rest of continental Europe. Those moves could be rendered useless if London no longer acts as a bridge to the EU.

"A lot of the banks have used Britain as sort of the shoehorn into the continent," said John Manley, chief equity strategist at Wells Fargo Funds Management. "International banks are going to have to rejigger their operations."

For European coverage, Manley said firms will likely move employees to other cities like Dublin or Frankfurt, for the same European access that London once offered.

It's a consideration that's affecting some smaller firms as well. Seattle-based portfolio management firm Smead Capital, for example, was planning to market a fund in mainland Europe and had one employee waiting for a visa and looking for office space. But the June 23 referendum has complicated plans.

"If they Brexit, then London is a lousy place to call on Europe," said Bill Smead, founder and CEO of the firm. "If the bank has employees in London as a base to deal with mainland Europe, those employees have to be moved to mainland Europe."

J.P. Morgan CEO Jamie Dimon is dealing with the issue on a much larger scale. The bank has more than 16,000 employees in Britain, and Dimon warned that a Brexit would force the bank to move a quarter of those jobs from the U.K., calling it a a "terrible deal" for the British economy, according to Reuters. Other banks, from the U.S. and elsewhere, have echoed Dimon to one degree or another, including HSBC, Deutsche Bank, Citigroup, and Morgan Stanley.

Global uncertainty has been a drag on the banks so far this year, and next week may bring the uncertainty to a boil. The Federal Reserve will announce the results of annual stress tests on 33 of the largest U.S. banks, required by the 2010 Dodd-Frank Act, on June 23, the same day as the Brexit vote. A week later, the Fed discloses whether or not it has approved capital plans of the largest banks, including whether or not they can increase dividend shares and buybacks.

"It's a perfect storm of uncertainty on banks," Smead said.

For some U.S. companies, it's not just parts of operations but whole headquarters that have moved from the United States to the United Kingdom. The world's largest insurance broker, Aon, moved its corporate headquarters to London from Chicago in 2012. The move was meant to give the company more access to emerging markets through London, "a key international hub of insurance and risk brokerage," the company said in a statement.

"If Britain votes to leave the European Union, the innovative center of excellence that has set London apart in the insurance space will be deeply challenged," Aon told CNBC in a statement. "Talent is a true differentiator for the city of London, and to create a barrier between the industry that addressees the world's most complex risks and the global talent needed to do this will have real implications."

Companies with the highest sales exposure to the U.K. include eBay, Ford, and Xerox, according to data from Bloomberg and J.P. Morgan. Among the companies cited in the data, Penske Automotive Group gets the largest chunk of its revenue from Britain — 33.4 percent.

Xerox disputed the revenue figure from Bloomberg and J.P. Morgan, telling CNBC after the Brexit vote that the portion of its revenue from the United Kingdom is much lower — 5 percent. Another firm, PRA Health, also disputed the validity of the study's numbers, but told CNBC that it does not disclose revenue on a per-country basis.

American companies obviously report earnings in dollars, and they could suffer from stumbles in the British pound as they lose money on the conversion back into their own domestic currency.

Pound sterling has fallen 10 percent against the dollar in the past year and is down nearly 5 percent year to date. A stronger dollar has already undercut earnings and revenue in large-cap companies that rely on sales from abroad.

"As an immediate risk, we're looking at a fairly significant downside in the pound sterling, and quite likely a higher U.S. dollar," said Shaun Osborne, chief FX strategist at Scotiabank. "It's unlikely to be contained to the pound sterling, there's probably going to be negative fallout on the euro as well."

Many American firms that import into or export from the UK are likely to be stifled by renegotiated deals and restrictions. A number of American companies use the United Kingdom as a base of European operations.

"There will likely be tariffs to pay for cross-border market access," said Matthew Beesley, portfolio manager and head of global equities for Henderson Global Investors. "It is to be negotiated, so we just don't know. This is the uncertainty."

Coca-Cola Bottling, which manufactures in the U.K., international retailers like Abercrombe & Fitch and Gap, Delfy, Invesco, and Walmart are also heavily exposed to Britain and likely to suffer in the event of a Brexit, Beesley said.

Ford is another company at risk, with more than 18 percent of revenue, according to the Bloomberg and J.P. Morgan data, coming from the U.K.

"We're the number one brand in the U.K., and for us the uncertainty which we've seen in the currency is a significant issue for our 14,000 employees," Jim Farley, Ford's CEO for Europe, the Middle East and Africa, told CNBC this week.

James Quincey, Coca-Cola's president and CEO also joined the choir of executives urging against a Brexit. Mark Dorsett, director of Caterpillar in the U.K. said British membership is fundamental to their business interests, and signed a letter titled "Brexit would hit UK growth and impede foreign investment." CEOs of GE and EMC also signed the piece, published in the Financial Times.

Companies with a domestic orientation could still be affected by a Brexit, although they have no notable exposure overseas.

"Bottom line is it's not just a U.K. story," said David Rosenberg, chief economist and strategist at wealth management firm Gluskin Sheff. "Bigger picture, it's going to usher in a period of intense uncertainty in a critical chunk of global GDP at a time when growth is already so feeble."

Rosenberg highlighted "viral" implications of a Brexit — it stands to make investors more wary of international investments generally — and the importance of Europe to the world's economy. The European Union accounted for 16.9 percent of the world's gross domestic product based on purchasing-power-parity (PPP) in 2015, while the United States made up 15.8 percent, according to data from the International Monetary Fund.

For those with cash to put to work, Rosenberg advises staying local. "You want to be involved in companies with a more local than international flavor," he said.

All of that said, Wells Fargo's Manley said they'll find a way to be flexible.

"It'll be a nasty process, but not particularly devastating," Manley said, adding that lawyers stand to make a fortune from the paperwork alone, if a Brexit happens. "There will be greater costs as they adjust to this, but it won't necessarily be a problem for their businesses."