Bank of America was fined 13.3 million pounds ($20 million) by the British markets regulator for failing to properly report millions of transactions, the highest such penalty imposed by the U.K. to date.

Merrill Lynch International, a unit of the U.S. bank, incorrectly reported 35 million transactions and failed to report another 121,387 transactions between November 2007 and November 2014, the Financial Conduct Authority said Wednesday. The bank had been reprimanded on the issue before, receiving a private warning in 2002 and a 150,000-pound fine in 2006, the FCA said.

"The size of the fine reflects the severity of Merrill Lynch's misconduct, failure to adequately address the root causes over several years despite substantial FCA guidance to the industry and a poor history of transaction reporting compliance," the regulator said in the statement. "Accurate and timely reporting of transactions is crucial for us to perform effective surveillance for insider trading and market manipulation."

Britain's FCA has been cracking down to ensure firms provide accurate and full reporting on transactions. It's fined a dozen banks now for these failures, including Barclays Plc, Credit Suisse Group AG, Commerzbank AG, Deutsche Bank AG and Royal Bank of Scotland Group Plc.

"While regrettable, today's decision principally refers to self-identified issues which we have sought to remediate as quickly as possible. We can confirm that no clients were financially impacted as a result," Bank of America spokeswoman Alexandra Parry said in an e-mail. "We are wholly committed to complying with all FCA requirements and continuously seek to improve all necessary aspects of our reporting."

Merrill Lynch agreed to settle at an early stage of the investigation and received a 30 percent discount on the fine, the FCA said. Without this discount the fine would have been 20 million pounds.

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