US banking giant Bank of America has been fined a record £13.3 million for failing to properly report millions of transactions, British regulators said Wednesday.

The penalty, equivalent to $20 million or 18.6 million euros, was handed down to Merrill Lynch International -- the group's corporate and investment banking division -- by London's Financial Conduct Authority (FCA) watchdog.

The FCA said in a statement that the division had incorrectly reported 35 million transactions and failed to report another 121,387 between 2007 and last November.

The regulator added that the record fine for failing to properly report transactions reflected the severity of the lender's misconduct and its failure to adequately address the root causes over several years, despite a previous warning in 2002 and a fine in 2006.

"Proper transaction reporting really matters," said Georgina Philippou, the FCA's acting director of enforcement and market oversight.

"Merrill Lynch International has failed to get this right again -- despite a private warning, a previous fine, and extensive FCA guidance and enforcement action in this area," she added in the statement.

"The size of the fine sends a clear message that we expect to be heard and understood across the industry."

The FCA has already fined 11 other firms for transaction reporting breaches, including Deutsche Bank, Barclays, Credit Suisse and Royal Bank of Scotland.

Merrill Lynch agreed to settle at an early stage of the investigation. As a result, it received a 30-percent discount on the fine which would otherwise have been £20 million, according to the FCA.

In response, a Merrill Lynch International spokesman said that the group had sought to address the issues.

"We are wholly committed to complying with all FCA requirements and continuously seek to improve all necessary aspects of our reporting," he said.

"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."

BoA is second-biggest US bank by assets, and is based in Charlotte, North Carolina.