Wall Street is used to getting the opportunity to influence bank rules before they are unveiled. Now financial firms are getting the chance to argue that a key capital requirement should be softened even after it was supposed to be finished.

A global panel of regulators that includes the U.S. Federal Reserve and Bank of England will let the world's largest banks provide additional feedback on a rule released in January that forces lenders to have sufficient capital to back bonds, derivatives, and other securities they intend to trade, according to three people with knowledge of the matter. Scheduled to take effect in 2019, the directive seeks to ensure that banks can weather plunges in asset values.

With the ink barely dry, banks plan to air their grievances at a meeting in London next month, said the people who asked not to be named because the planned discussions are private. The talks are slated as regulators, particularly in Europe, signal an increased willingness to assess the impact of dozens of rules put in place since the 2008 financial crisis. While more capital makes banks safer, it's a tough pill to swallow for European lenders that are struggling to make money.