Citigroup, Goldman, Morgan Stanley have most dollar exposure to Brexit risk

At a time when banks face pressure on a number of fronts, the Brexit situation in Europe doesn't help.

Regulatory issues as well as a low interest rate environment that looks like it could last for years to come are just two unfriendly forces against the institutions, which as a group have lost more than 11 percent year to date, as measured by the KBW Nasdaq Bank index.

Now, disruptions in Europe with the potential that Britain may leave the European Union could cause more trouble.

Just how much, though, is hard tell. Fitch Ratings believes some damage could come, but there also will be opportunity.

"A lot of the U.S. banks in Europe have consolidated their operations in the U.K., so they've gotten a lot of cost efficiencies out of that," Joo-Yung Lee, head of North American financial institutions at Fitch Ratings, told CNBC. "With the Brexit, we think there will be some restructuring changes ahead of them in terms of potentially having to pivot away from the U.K. So clearly it depends on how the trade agreements are worked out."

One area banks could benefit is from the associated volatility. The Dow industrials lost more than 800 points in a two-day period following the vote, but have been in rally mode the past three sessions.

"Volatility does help the banks, as long as it's not too much volatility that keeps investors on the sidelines," Lee said. "What were are seeing is there has been quite a bit of volume, so that is good for the banks."

At an institutional level, Citigroup has the highest dollar exposure to the U.K. at $118.9 billion, However, that's only a fraction of the bank's $1.8 trillion in assets and just 16.4 percent of its non-U.S. exposure, according to S&P Global Market Intelligence. Other financial institutions, though, face higher risks.

S&P compiled a list that provides perspective on total exposure:

US banks in the UK

Company	UK exposure ($B)	% of total non-US exposure)
Citigroup	118.9	16.4
Goldman Sachs	103.9	29.4
Morgan Stanley	79.2	26.6
Bank of America	70.2	29.4
State Street	19.8	25.8
Bank of New York Mellon	18.1	26.2
Northern Trust	9	37.3
American Express	5.6	34.3
Capital One	3.7	41.8
United Services Auto Association	1.8	59.2