Six of the top financial institutions in the U.S. together used nearly 400 recognized tax havens in 2014. Their offshore cash totaled $126 billion. Mike Ehrmann/Getty Images

Six of the nation's largest banks -- Bank of America, Citigroup, JPMorgan Chase, Wells Fargo, Morgan Stanley and Goldman Sachs -- utilize a combined 395 known tax havens to avoid potential tax bills in the billions, according to a new analysis.

In total, the big six banks kept $126 billion in cash or cash equivalents in recognized tax havens like Bermuda, Mauritius and Luxembourg.

Those holdings comprise 6 percent of the $2.1 trillion held offshore by U.S. corporations in the Fortune 500. Nearly three-quarters of companies on the list utilize offshore tax havens, according to a study from the left-leaning nonprofits Citizens for Tax Justice and the U.S. Public Interest Research Group.

The problem, according to the study's authors, is that overseas vehicles allow the country's largest corporations to avoid massive tax obligations that eventually fall on the taxpaying public.

"Every dollar in taxes that corporations avoid by using tax havens must be balanced by higher taxes on individuals, cuts to public investments and public services, or increased federal debt," the authors wrote.

Of the major financial firms analyzed in the report, Wells Fargo operates the most offshore subsidiaries: 210. Yet the firm pays the highest effective tax rate of its peers -- 24 percent -- on all of its overseas cash, suggesting that much of its assets sit in jurisdictions with tax laws similar to those in the United States.

Citigroup, meanwhile, pays an effective tax rate of just 9 percent on its $43.8 billion in offshore lucre -- roughly one-quarter of the U.S. corporate tax rate. Wells Fargo’s offshore tax bill came to just 6 percent last year.

The financial giant’s overall offshore holdings still fall short of Apple, whose $181 billion in overseas cash tops all American corporations.

The report comes as governing bodies around the world attempt to plug holes in porous tax regimes and clamp down on multinationals that shift profits to more-forgiving jurisdictions. This week the Organization for Economic Cooperation and Development, the research organization of the world’s richest economies, estimated that between 4 percent and 10 percent of global corporate tax revenues leak away in offshore accounts.

Matching similar efforts from the European Union, the OECD recommended new rules that would require multinational corporations to disclose in more detail where they route their profits.

And there’s evidence that major banks have consolidated their overseas subsidiaries. According to a 2008 Government Accountability Office report, Citigroup had 427 offshoots in tax haven jurisdictions that year, more than 10 times the number listed in the recent study. JPMorgan Chase similarly reduced its tax-haven subsidiary count from 50 in 2008 to four in 2014.

Across the American corporate world, however, overseas cash hordes have only grown, doubling between 2008 and today. According to some estimates, the total lost revenue in corporate taxes amounts to $90 billion annually -- about one and a half times the yearly cost of tuition at all public colleges.