For the foreign-exchange sales team in Citigroup Inc.'s London office, Dodd-Frank regulations mean extra hours at work.

At least two members of staff have been staying until after 9 p.m. because some clients are no longer allowed to deal with Citigroup colleagues in New York, Alex Jackson, head of European investor sales, foreign exchange and local markets, said in a phone interview on Oct. 25. That's because the Dodd-Frank Act prevents people in the U.S. from trading with counterparts who haven't agreed to International Swaps & Derivatives Association rules, Jackson said. European money managers and Brazilian hedge funds are among customers relying on the arrangements, he said.

"No non-compliant investor or client is able to trade with a U.S.-based salesperson or trader physically located in the U.S.," London-based Jackson said, citing a footnote to the regulations on swaps trading. "Any client who has not signed the ISDA protocol falls under this."