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With a referendum looming on whether Britain will leave the European Union, the country’s voters seem sharply divided on whether to stay or go.

But for the big banks that dominate the City, as London’s financial industry is known, there is little disagreement: They don’t want to risk going it alone.

London is a major financial gateway, the biggest and busiest in Europe and rivaling Wall Street as a hub of international trading in stocks, bonds, currencies and commodities. In the extreme, banking executives worry about a painful economic and financial fallout from a British exit — known, in the political shorthand, as a Brexit.

Britain Receives Proposals for ‘Better Deal’ to Stay in the E.U.FEB. 2, 2016 David Cameron Renews Push to Reach E.U. Membership DealJAN. 29, 2016 “London is a global center; it’s also a European center — one is contingent on the other,” said Alan Houmann, managing director of European government affairs for Citigroup. “Financial institutions come into London from third countries, like the U.S. and from the rest of the E.U., so this creates this huge ecosystem at which London sits at the center.”

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Paul Lynam is the chief executive of Secure Trust, a small British bank that has little to gain commercially from the European Union’s single market. Credit Tom Jamieson for The New York Times Prime Minister David Cameron, who wants his country to remain in the bloc, is negotiating changes to the country’s terms of membership ahead of a summit meeting with European leaders on Feb. 18 and 19. (The referendum in Britain could come as soon as June.) With his counterparts in Europe, Mr. Cameron is trying to secure Britain’s status in the financial hierarchy by seeking assurances that banks and other firms will not be discriminated against as the eurozone becomes more integrated.

He is navigating a political minefield. As much as Germany and France want Britain to stay part of a club that has helped reinforce peace and regional stability for decades, other members worry about the precedent of granting any one country too many side deals or breaching fundamental principles of unity.

And the big banks are hardly a sympathetic cause, making industry executives somewhat more tempered in their comments on a Brexit. The British public blames highly paid bankers for the financial crisis and the scandals over currency trading and interest-rate rigging.

British businesses of all sorts benefit from being able to buy and sell in the European Union’s estimated $15.1 trillion common-market economy and its 500 million people. And though immigration is a contentious issue, British companies benefit from their ability to hire any European Union citizen.

Bankers do not exactly love the raft of regulations in Europe, like the cap on bonuses and a proposal to tax financial transactions. But many believe that a Brexit would be problematic.

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A big concern is that a Brexit could set off an economic downturn for Britain and a plunge in the country’s currency. Financial services represent more than 14 percent of Britain’s nearly $3 trillion economy, according to the City of London Corporation, the de facto municipal government for much of the financial district.

Were Britain to leave the European Union, bankers in London might no longer have the automatic right to provide services to clients in other European nations — like securities trading, foreign exchange and investment management.

Britain would also cede its ability to help shape, block or amend European financial and banking laws. It could be demoted, in the catchphrase of anti-Brexit campaigners, from being a rule maker to a rule taker.

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Alan Houmann, a Citigroup executive, said London sits at the center of a financial ecosystem. Credit Tom Jamieson for The New York Times “Banks won’t disappear from London overnight, but they will over time if Britain votes ‘no,’ ” to the European Union, Michael Sherwood and Richard Gnodde, the co-chief executives of Goldman Sachs International, argued in an op-ed article in The Times, the London newspaper.

Goldman has been more visible than many big banks on this issue and has donated a six-figure sum to Britain Stronger in Europe, a lobbying campaign that is opposed to a Brexit. Goldman declined to comment on its donations.

Even bankers who operate solely within Britain acknowledge the potential disruptions.

Secure Trust, a small, domestically focused British bank, has little to gain commercially from the European Union’s single market. The bank’s chief executive, Paul Lynam, is also unhappy with the way European regulators interpret international rules. He notes, for example, that the European Union requires Secure Trust to maintain capital reserves — cushions against risk — that he says are larger than necessary for an institution like his that mainly serves consumers and small businesses.

Although he believes that a Brexit would be “made to work,” it would most likely mean years of negotiation over the details of Britain’s new relationship with the bloc. This process, he said, would be “messy, costly, disruptive and time-consuming.”

“We need to know what are the options being offered to us by way of concessions, relative to the dangers of the leap into the unknown,” Mr. Lynam said.

He is also concerned about the backlash among the banks’ customers. Most opinion polls show that the British public is almost evenly split on the Brexit issue.

“If I take a public position I will upset half my customers — a quarter of a million customers,” said Mr. Lynam. “So I won’t be in a rush to do that.”

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Prime Minister David Cameron during an official visit to Poland last week in Warsaw. Credit Radek Pietruszka/European Pressphoto Agency Still, some in the City have openly contemplated a Brexit.

One is Norman Blackwell, a former adviser to two Conservative prime minsters, John Major and Margaret Thatcher, who is now chairman of Lloyds Banking Group. Last year, Mr. Blackwell told the House of Lords — where he is a member — that Britain’s relationship with the bloc might become “increasingly unsustainable.” He noted that his comments reflected his personal views.

And some hedge fund executives in the City would just as soon quit the European Union. Hedge funds have tended to chafe under the closer oversight that European regulators have demanded since the global financial crisis of 2008.

Crispin Odey, a leading figure in London’s hedge fund world, has donated to Vote Leave, a lobbying group campaigning for a Brexit. Mr. Odey has said he “would rather we were out of Europe.”

But among the City’s hedge funds, there is no consensus, particularly given that they would still face European rules to court European business.

“Hedge fund businesses in the U.K. recognize that they would not be able to avoid E.U. regulation if there were a Brexit, since all non-E.U. hedge funds today have to comply with a range of E.U. rules in order to raise capital in the E.U. or trade on the bloc’s capital markets,” said Jack Inglis, chief executive of the London-based Alternative Investment Management Association, a trade group.

Even the big international banks that benefit most from the European single market assume they would probably survive a Brexit. Most have operations in the eurozone that could be expanded.

Some are already moving some operations from London to reduce costs. Credit Suisse, for example, opened a branch office in Dublin.

Seen in that light, a Brexit is not as risky to any specific bank as it is to London’s role as a global financial center.

“If no one moves, then no one moves,” Mr. Houmann of Citi said. “If everyone moves a bit, then there’s quite a big drop.”