LONDON (AP) -- Gina Miller needs to look no further than her own small investment firm to decide that leaving the European Union would be bad for Britain.
She has an analyst who is Italian: would he need a visa? She has customers who are British retirees living in Spain: will they return home to keep access to health care and liquidate their euro investments? Her firm, SCM Direct, works closely with French bank Societe Generale: would that relationship continue?
"It would be a nightmare," she said suggesting the impact could be similar to the 2008 financial crisis. "Why would we do this to our country again? And this time it would be of our own making."
From the international banks in the skyscrapers of Canary Wharf to the traditional home of Britain's financial industry in the City of London and the hedge funds of Mayfair, bankers and money managers across the capital await the June 23 referendum on EU membership with trepidation. Many fear a vote to leave would undermine London's position as the world's pre-eminent financial center and damage an industry that underpins the British economy.
Jamie Dimon, chief executive of U.S. banking giant JPMorgan Chase, underscored those concerns last week when he appeared alongside U.K. Treasury chief George Osborne to make the case for remaining part of the EU single market, which with 500 million people is the world's biggest economy. In case of a British exit, or Brexit, from the EU, JPMorgan would have to move staff to the continent to ensure it could continue to serve clients who want to invest there, Dimon said. Other global banks with customers in the rest of the EU would be in a similar situation.
"A vote to leave would be a terrible deal for the British economy," he said. "At a minimum, a Brexit will result in years of uncertainty, and I believe that this uncertainty will hurt the economies of both Britain and the European Union."
Britain has been the gateway to the EU for many banks, brokerages and fund managers for decades. In addition to having a trusted legal system and institutions that operate in English, the language of international finance, London is in the right time zone to access most of the Earth during its working day and has a reputation for delivering top-notch financial services. The industry is also surrounded by an ecosystem of expertise — lawyers, accountants and consultants — to support it.
Some 60 percent of all European headquarters of non-EU firms are based in the U.K., according to TheCityUK, which lobbies on behalf of the financial industry. The U.K. hosts more headquarters of non-EU firms than Germany, France, Switzerland and the Netherlands put together.
London's advantages are such that people in favor of leaving the EU, such as Peter Hargreaves, co-founder of brokerage firm Hargreaves Lansdown, think it will retain its luster no matter what. He poured scorn on the notion that it could be easily replicated.
"In addition to the cost of building the infrastructure for a rival to the City of London one has also got to work out whether people will want to live in another financial center," he said. "They certainly wouldn't want to live in Paris —although Paris is a pretty city — purely because of the tax rates, and in all honesty they wouldn't want to live in Frankfurt because actually there isn't even the housing there to house them. People want to live in London."
London's financial sector has complained about a number of EU rules, such as limits on bankers' bonuses and an attempt to impose a tax on financial transactions.
Those considerations, however, are largely trumped by concerns that leaving the EU would make access to the other 27 EU countries more difficult, many analysts say.
The principle of "passporting" currently allows any firm registered in one EU country to operate in any other member state without facing another layer of regulation. It's the same principle that allows exporters to ship their goods to any EU country free of tariffs. Losing that freedom is a particular concern for the many foreign firms who use London not only as a financial hub but as an entry point into the EU.
"I can treat a customer in France or Germany or Italy exactly the same way I can treat a customer in Birmingham. That is extremely rare," said Phillip Souta, head of U.K. public policy at the global law firm of Clifford Chance.
While the U.K. could probably negotiate a new arrangement for trade in goods, it would be much more complicated to hammer out a deal on services, said Angus Armstrong, chief of macroeconomics at the National Institute of Economic and Social Research. The situation has no precedent — no country the size of the U.K. has ever left such an integrated economic union — so the outcome of any talks can't be predicted.
And anything that curtails Britain's financial industry has implications for the U.K. economy as a whole, not just the bankers who were pilloried for taking home million-pound bonuses while they fueled the global financial crisis.
TheCityUK notes that the sector supports the economy by providing financing for businesses, overseeing retirement savings, providing mortgages and making insurance payments. Related professional services include legal, accounting and management consulting firms.
The financial sector accounts for 11.8 percent of economic output and employs 2.2 million people, or 7 percent of the nation's workforce, according to TheCityUK.
The industry's importance is even more obvious in Britain's trade figures. While the country posted an overall trade deficit of 34.4 billion pounds ($50 billion) in 2014, it generated a 72 billion pound surplus from exporting financial and related services.
"I don't think people quite understand what goes on in the City," said Vicky Pryce, an economist and former joint head of the U.K. government economic service. "A strong financial sector employs a lot of people and is vital for the health of the economy."
The City of London — the square mile roughly bounded by the walls of the original Roman city — has been the financial heart of Britain since the 17th century, when merchants meeting at local coffee houses formed Lloyd's of London and the London Stock Exchange. It has survived wars, famines and fires and would likely survive any outcome from the EU vote. The question is in what form.
Jeffrey Evans, the Lord Mayor of the City of London, whose office has promoted The City as place to do business since the Magna Carta was signed in 1215, says leaving the EU in not a risk worth taking.
Speaking in an opulent drawing room that feels more like a museum, with Dutch masters lining the walls, Evans noted that leaving the EU would be irreversible.
"It's a very important matter," he said. "This is a decision that will affect future generations."