The race is on to be the new London.

Unless Britain finds a way to undo its decision to leave the European Union, London’s days as the pre-eminent global financial capital, ranked even ahead of New York, may be numbered.

I spoke this week to several high-ranking executives at major financial institutions that collectively employ tens of thousands in London. While none of them have any immediate plans to move their European headquarters from Britain’s capital, all agreed they would eventually shift a significant number of highly paid employees to cities that remain in the European Union.

One executive in charge of relocation (who like the others, spoke only on condition of anonymity because of the political sensitivity of the issue) said the percentage of employees in his firm who might be required to move ranged from 10 percent to 40 percent. “Multiply that throughout the industry and it’s tens of thousands of people and their families,” he said. “And bear in mind that most of these people are millionaires.”

Others said it would take five to 10 years, but a new London would almost certainly emerge in one of the other prominent cities of the European Union. “When I moved to London years ago, it wasn’t exactly cosmopolitan,” said another executive. “It wasn’t a place for great restaurants. The infrastructure has improved dramatically. It will take time, but eventually one big hub will develop.”