The Royal Bank of Scotland failed to properly report 44.8 million transactions between 2007 and 2013, the Financial Conduct Authority has said.
The FCA also said the bank "failed altogether" to report 804,000 transactions in the same period - between November 2007 and February 2013.
The figures meant 37% of relevant transactions carried out by RBS in the period were not properly reported, the FCA said.
More top news
Very mild for mid February
Five teenagers have been arrested in London on the suspicion of preparation of acts of terrorism.
Goals from Lucas Perez and Theo Walcott ended Sutton United's FA Cup dream on Monday night.