The head of U.S. bank JPMorgan Chase told British staff on Friday a decision by Britain to leave the European Union could mean "fewer" jobs with the bank there and more jobs in Europe.
"A vote to leave would be a terrible deal for the British economy," Chief Executive Jamie Dimon said, speaking alongside finance minister George Osborne in Bournemouth, where the bank employs 4,000 out of 16,000 staff based in Britain.
"So if the UK leaves the EU, we may have no choice but to reorganize our business model here. Brexit could mean fewer JPMorgan jobs in the UK and more jobs in Europe."
Campaigners seeking to win a vote for Britain to remain in the EU in this month's referendum have said London's financial sector could lose out if the world's fifth-biggest economy leaves the EU, with concerns that Britain's role as a global financial hub and major trading center for the euro could be under threat if it gives up its membership.
Bank of England Governor Mark Carney has said some big financial firms might move their business out of Britain if the country did not secure the same kind of access it currently has to the EU.
"One realistic outcome is that we lose the ability to passport our banking and trading services into Europe," said Dimon.
"But our clients will still need us to trade within what will then be the EU. If that's what the rules say, we will need to do what works."
The five largest U.S. banks employ 40,000 people in London, more than in the rest of Europe combined, taking advantage of the EU "passporting" regime that allows them to offer services across the bloc out of their British hubs.
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.