“The European Union is very much built on mutual trust, the relationships between leaders and the bonds of different countries,” Mr. Cramme said. “Cameron has taken a very tough line, and this will backfire.”

There was widespread confusion over how this would all play out. One view was that no matter what Mr. Cameron says, Britain will still be subject to European regulations, at least as far as Europe has jurisdiction over its non-British banks. Others doubted that the euro zone would long stand for having its main financial center in a nonmember country, and said it would inevitably begin directing its banking activities to Frankfurt or some other financial capital.

“The concern is that if the U.K. finds itself increasingly isolated,” said Bob Penn, a partner at the law firm of Allen & Overy. “You can see — if you’re a pessimist — that Europe could get its revenge on the U.K. by a whole array of bureaucratic and regulatory reforms.” In that scenario, Continental banks would be prohibited from dealing with the City unless the British firms adhered to Europe’s regulations.

Down that road lies Britain’s possible exit from the European Union, a prospect that many British Conservatives find positively alluring. “There is also an exciting sense that we are at the beginning of a very profound change in which our tortured relationship with Europe will be redefined, at long last,” said The Daily Mail.

An American banking industry official said he would not totally brush aside the possibility of retaliatory actions by Germany and France, but he said such responses were unlikely. After all, those countries have a long history of championing their global banks and would probably want to continue having an outsize presence in the City.

“The City has huge benefits for the E.U., and it’s not in the euro zone’s interest to see that evaporate and moved somewhere else,” Yael Selfin, a director at PricewaterhouseCoopers, said. “This is all part of a much longer bargaining process.”

London, many finance experts noted, is more an international financial center than a purely European one. Even if it were excluded from the euro zone, it would retain many of the attractions that have led Asians and Americans to locate major branches in London and to cut their deals by the Thames.