A British exit from the European Union could wreck London's position as the only financial center to rival New York and isolate the country's economy, research ordered by a lobby group for banks and money managers showed.
Prime Minister David Cameron has promised to renegotiate the terms of Britain's EU membership and hold an "in-out" referendum by the end of 2017 if his Conservatives win a 2015 national election.
But many of the most powerful banks, insurers and money managers in the City of London are increasingly concerned that Cameron's gamble could allow the country's $2.5-trillion U.S. economy, the world's sixth largest, to slip out of the EU.
TheCityUK, whose members include asset managers, banks, insurance and accountancy firms, warned that Britain outside the EU would be shorn of influence, less attractive to investors and vulnerable to regulations over which London had no influence.
London dominates the $5-trillion-U.S. a-day foreign exchange market, trading twice as many dollars as the United States and more than twice as many euros as the entire euro zone, according to the lobby group.
A growing number of banks, including Goldman Sachs, Citi and JPMorgan, have warned a "Brexit" could hurt London's position.
Opponents of the EU say Britain would do better to trade with the world from outside the bloc. Opinion polls show voters are split on the issue, with 40% wanting to remain in the EU and about the same proportion saying they would opt to leave.
One law firm said its research showed that under five possible scenarios for Britain leaving the European Union, the financial services sector - which accounts for about one-10th of Britain's gross domestic product - would be harmed.
While Cameron has pledged to hold a referendum if he wins in 2015, the opposition Labour Party has said any Labour government would be unlikely to hold such a vote this decade.
