UBS has been fined £27.59m by the City watchdog for misreporting 135.8m transactions.
The Swiss investment bank failed to provide complete and accurate information in relation to 86.6m reportable transactions, and also wrongly reported 49.1m transactions over a period of nine years and six months, the Financial Conduct Authority (FCA) said.
An investigation by the FCA also found that UBS had failed to take reasonable care to organise and control its transaction reporting methods, which are supposed to help the regulator identify potential market abuse and fight financial crime.
Mark Steward, FCA executive director of enforcement and market oversight said: "Firms must have proper systems and controls to identify what transactions they have carried out, on what markets, at what price, in what quantity and with whom.
"If firms cannot report their transactions accurately, fundamental risks arise, including the risk that market abuse may be hidden."
The fine was reduced by 30 per cent from £39.4m after UBS agreed to resolve the case.
The FCA has fined 12 other banks for transaction reporting breaches including Merrill Lynch, Deutsche Bank, Royal Bank of Scotland and Barclays.