Former New York mayor Michael Bloomberg says that a Brexit vote on 23 June could hit UK-EU trade and leave British-based "employees worse off".
He warns financial jobs could leave London for Paris or Frankfurt.
Writing in The Times, Bloomberg says "a vote to leave is a risk" and asks "is the risk worth taking?"
On the other hand, Leave campaigners said the EU benefited "big businesses and fat cats who care about their bonuses" but did not "work for the British people".
Bloomberg's name also appears on a letter, signed by a number of the major multinationals investing in the UK, warning of Brexit dangers.