LONDON — Bank of America was fined $20 million by the British markets regulator for failing to properly report millions of transactions, the highest such penalty imposed by Britain 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, Commerzbank, Deutsche Bank and Royal Bank of Scotland.

Merrill Lynch agreed to settle at an early stage of the investigation and got a 30 percent discount on the fine, the FCA said.

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