American Express Co.'s foreign exchange international payments unit offered low currency conversion rates to its clients, and then raised conversion rates without notifying clients in order to increase revenue and employee commissions, The Wall Street Journal reported, citing people familiar with the matter.

The practice, targeting small and midsize business, dates back to at least 2004, the news outlet reported.

American Express' spokeswoman Marina Norville told the news outlet that American Express does not have contractual pricing arrangements with most of its foreign-exchange clients. The company will conduct a review with an external party, Norville said.

The commission-driven culture of American Express' foreign exchange business encouraged the practice, a current and former employee of American Express told the Journal. However, managers in the division have stopped the practice in recent months, the news outlet added, citing former and current employees of the company.