LONDON — Score one for Frankfurt.

With Britain locked in negotiations with the European Union over their post-“Brexit” relationship, the financial companies that dominate London’s economy have forged ahead, eager to ensure they can continue to serve clients across the Continent. That has set other European cities scrambling to win a slice of the business that the British capital has long kept for itself.

The American banking giant Citigroup is taking a step toward reducing its dependence on London and will open a second trading hub in Frankfurt, said a person familiar with the bank’s plans, who was not authorized to speak publicly about them and so spoke anonymously. The bank has had an operation in Frankfurt for decades, but it would expand its business there with this latest decision.

The development is largely symbolic, with a relatively small number of staff members in London expected to move to Germany. But it is another trickle of job losses that some British business and political leaders fear could turn into a flood in the coming months and years, as the full picture of the nation’s future trading relationship with the European Union becomes clear.

Britain’s membership in the 28-nation bloc has bolstered London’s position as a global financial hub in part because European rules allow “passporting,” under which a banking license in a single member-state allows a lender to work throughout the European Union.