The Royal Bank of Scotland (RBS) has been fined £5.6 million ($8.6 million) for not properly reporting close to 45 million trades between November 2007 and February 2013.
During this period the Financial Conduct Authority (FCA) found mistakes in over a third of the bank's transactions, which included failing to identify the parties in a trade and using the wrong pricing and dates.
On top of this, of the 44.8 million misreported trades, 804,000 of them were not reported at all.
This took place under the regimes of both Fred Goodwin, who has left the company, and Steven Hester, who will soon be departing.
Tracey McDermott, the FCA's director of enforcement and financial crime, said: "These failures are particularly concerning because the FCA already provides extensive guidance to firms on how to submit and check these reports."
As RBS decided not to appeal against the FCA's decision the fine includes a 30 per cent reduction.
By Gary Cooper