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.
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