The Financial Conduct Authority (FCA) has fined Merrill Lynch International £34.5 million for failing to report transactions.
The bank has been accused of failing to report 68.5 million exchange traded derivative transactions between 12 February 2014 and 6 February 2016.
Merrill Lynch International (MLI) was the subject of two earlier and related transaction reporting cases, the FCA said.
Back in 2015 the firm was fined £13.3 million for incorrectly reporting over 35 million transactions and failing to report another 121,387 transactions between 2007 and 2014.
In 2006, the company also received a fine of £150,000 by the Financial Services Authority.
This is the first enforcement action against a firm for failing to report details of trading in exchange traded derivates under the European Markets Infrastructure Regulation (Emir).
Because MLI agreed to settle at an early stage, it received a 30% reduction in the overall fine. If this had not happened, the fine would have been £49.3 million.
Mark Steward, executive director of enforcement and market oversight, said: 'Effective market oversight depends on accurate and timely reporting of transactions. The obligations under Emir, as with Mifid, are key aspects of such oversight.
'It is vital that reporting firms ensure their transaction reporting systems are tested as fit for purpose, adequately resourced and perform properly. There needs to be a line in the sand. We will continue to take appropriate action against any firm that fails to meet requirements.'