Banks are adjusting contracts with “Brexit clauses” to protect themselves if the separation is chaotic. Lawyers are checking regulations, jurisdiction by jurisdiction, to gird for possible future contractual disputes.

Cities across the Continent have been vying for a piece of an industry that represents about 7 percent of Britain’s gross domestic product and more than a million jobs there.

Frankfurt, Paris, Dublin and Luxembourg will be the first to secure new business as financial services companies gauge how profitable London remains. In the next months, these cities, along with Madrid and Milan, will find more traders, compliance teams, human resource managers and technology workers in their midst. Amsterdam will become home to more European markets.

One big Brexit beneficiary is Dublin, where Bank of America, Citigroup and Barclays are expanding their ranks. “Dublin is our headquarters for our European bank now, full stop,” said Anne M. Finucane, vice chairwoman of Bank of America, which employs more than 800 people there.

“There isn’t a return. That bridge has been pulled up,” Ms. Finucane told the European Financial Forum on Wednesday . “From a trading perspective, likewise Paris would be the European trading arm.”