LONDON — A British regulator on Wednesday fined an arm of Bank of America Merrill Lynch 13.3 million pounds, or about $19.8 million, for failing to properly report tens of millions of client transactions over a seven-year period.

The Financial Conduct Authority of Britain said that Merrill Lynch International incorrectly reported details regarding client transactions ranging from the identity of counterparties to the trading times of some transactions. The fine related to more than 35 million transactions between November 2007 and 2014.

Merrill Lynch also didn’t report another 121,000 transactions at all during the period, the regulator said.

The fine was the largest the Financial Conduct Authority has ever imposed for such failures. The regulator’s predecessor had previously issued a so-called private warning to Merrill Lynch in 2002 and fined the company £150,000 in 2006.