A vote to leave would be particularly difficult for U.S. investment banks since most run the bulk of their European trading operations out of London offices, and some could even give up parts of their business in the bloc altogether, Reuters reported in May.
JPMorgan alongside Wall Street rivals Morgan Stanley, Citi and Goldman Sachs have donated six-figure sums to the campaign for the country to stay in the EU.
Dimon said in April that years of economic uncertainty would be the "best case" outcome from a decision by Britain to leave the European Union, in his annual letter to shareholders.
JPMorgan is the largest private sector employer in the south coast town of Bournemouth, where it operates a hub for technology, operations and client services and supporting processing activities.
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